The Stoics were prolific writers, especially Chrysippus (the third head of the school), but their writings have been almost completely lost.

The reasons for this loss are many, but perhaps the primary one is that Stoicism itself became less popular and by the middle of the third century AD no longer attracted practitioners. Without the school, the books went out of circulation. Further, for various reasons, it became possible to think that Stoic philosophy was a development of the philosophy of Plato and thus need not be understood on its own terms. (This process of looking back for inspiration to the classical philosophy of Plato and Aristotle seems to begin with Panaetius, who was a great admirer of Plato. Panaetius succeeded Antipater of Tarsus in 129 AD to become the seventh head of the Stoic school. Posidonius too was an admirer of Plato.) With Stoicism viewed in this way, the Platonists (whose school was then the dominant tradition) saw no need to preserve the surviving early Stoic texts since the Platonic texts were thought of as the best indication of the true philosophy Plato had glimpsed. The Stoic texts that did survive, primarily those of the later Stoics (Epictetus, Seneca, and Marcus Aurelius) were preserved in part because they seemed to serve a practical need consistent with the current world view.

Stoicism (which as a school of thought divides into early, middle, and late) takes its name from their initial meeting place in the Ποικίλη Στοά. A στοά is a roofed colonnade. The Ποικίλη Στοά was a particular roofed colonnade that derived its name from its murals. It was located at the north-west corner of the Agora, the central square in Athens.

Steps of the Poikile Stoa
West end steps of the Ποικίλη Στοὰ. In the foreground is part of the
foundation for the Hellenistic Gate, which allowed access to the Agora
from the north.

Early Stoicism

Zeno of Citium (334-262), founded the school in about 300
Cleanthes (334-230), succeeded Zeno
Chrysippus (279-206), third and most influential head of the Stoic school

Middle Stoicism

Panaetius of Rhodes (185-109)
Posidonius of Apameia (135-51)

Late Stoicism

Seneca (4 BCE-65)
Epictetus (55-135)
Marcus Aurelius (121-180), Roman Emperor from 161-180

The Soul in the Adult is Reason

The Stoics (like the Hellenistic philosophers generally) tried to correct what they saw as mistakes in the classical tradition of Plato and Aristotle. Plato and Aristotle thought that the soul has a part with reason and a part without reason and that Socrates had wrongly overlooked the part without reason in the way he thought about human beings. The Stoics, in contrast, thought that although a human being begins life with a soul without reason, this soul ceases to exist when a human being becomes an adult. (For the Stoics, adults are humans with reason. Children and non-human animals lack reason.) The Stoics thought that Plato and Aristotle failed to realize that the soul in the child is transformed and replaced so that in the adult all motivation is in terms of reason.

"Whereas the ancients claimed the passions (perturbationes) are natural and have nothing to do with reason, and whereas they located desire in one part of the soul, and reason in another, Zeno would not agree with that. He thought that these commotions were equally voluntary and arose from a judgment which was a matter of mere opinion..." (Cicero, Academica I.39).

(Marcus Tullius Cicero (107-44) was a Roman orator. After Caesar was assassinated in 44 BCE, Cicero tried but was unsuccessful in opposing Antony. Cicero himself was murdered not long after, but in the last years of his life, he turned to writing philosophy. (The dictatorship of Caesar had forced him out of public life, and his personal life was also in disarray. In 46, he divorced his wife (with whom he had a marriage of convenience) and married a younger woman. This marriage ended in less than a year, in part because of his grief over the death in childbirth of his daughter.) He decided that since he was unable to serve the public through politics, he would serve through education by writing philosophy in Latin. His writing is primary source for much of the thought of the Hellenistic philosophers.)

The Stoic Theory of Impressions

The Stoics developed their theory of “impressions” (φαντασίαι), and ultimately their theory of the will, to clarify what they understood as the Socratic idea that motivation has its basis in reason because it stems from beliefs (knowledge or opinion) about what is good and what is bad. Impressions are representations. They represent the world as being a certain way. In adults, because they have reason, impressions have a propositional structure. Unlike animals and children, adults have concepts. Their impressions of things are impressions of these things as things of a certain sort. The adult, for example, sees human beings as human beings. Animals and children lack reason. Their impressions are in terms of images and do not represent things are as things of a certain sort.

"An impression (φαντασίαν) is an imprint on the soul" (Diogenes Laertius, Lives of the Philosophers VII.45). "[Diocles of Magnesia says that the] Stoics agree to put in the forefront the doctrine of impression and sensation, inasmuch as the standard (κριτήριον) by which the truth of things is tested is generically an impression, and again the theory of assent and that of cognition (καταλήψεως) and thought, which precedes all the rest, cannot be stated apart from impression" (Diogenes Laertius, Lives of the Philosophers VII.49).

Diocles of Magnesia wrote a work entitled Survey of the Philosophers (Ἐπιδρομὴ τῶν φιλοσόφων) that Diogenes excerpts. Apart from Diogenes, nothing is known about him or his work.)

"Another division of impressions (φαντασιῶν) is into rational (λογικαί) and irrational (ἄλογοι), the former being those of rational creatures, the latter those of the irrational. Those which are rational are processes of thought, while those which are irrational have no name." (Diogenes Laertius, Lives of the Philosophers VII.51).

"An animal's first impulse, say the Stoics, is to self-preservation, because nature from the outset endears it to itself.... [T]hat pleasure is the object to which the first impulse of animals is directed, it is shown by the Stoics to be false. For pleasure, if it is really felt, they declare to be a by-product, which never comes until nature by itself has sought and found the means suitable to the animal's existence; it is an aftermath comparable to the condition of animals thriving and plants in full bloom. And nature, they say, regulates the life of plants too, in their case without impulse and sensation, just as also certain processes go on of a vegetative kind in us. But when in animals impulse has been superadded, whereby they are enabled to go in quest of their proper sustenance, for them, say the Stoics, the rule of nature is to follow the direction of impulse. But when reason by way of a more perfect leadership has been bestowed on the beings we call rational, for them life according to reason (λόγον) rightly becomes the life according to nature. For reason intervenes as the craftsman of impulse" (Diogenes Laertius, Lives of the Philosophers VII.85-86).

Impulsive impressions

The Stoics think that motivation works in terms of "impulsive impressions" (φαντασίαι ὁρμητικαί) and that these impressions function differently in adults and in animals and children.

To understand the Stoic view, it helps to consider a model in which maintenance and achievement goals explain behavior. A maintenance goal encodes a relationship with the world that an agent maintains through its behavior. When the relationship fails, the maintenance goal tends to give rise to an achievement goal. The achievement goal tends to cause the agent to engage in behavior that will reinstate the relationship. Consider hunger. When animals are hungry, they tend to move to find food and eat it. In terms of the model, the conditional "if I am hungry, I find food and eat it" is instantiated in the animal so that it functions as a maintenance goal. When the animal registers the truth of the antecedent, the content of the consequent is activated as an achievement goal.

This model provides a way to understand how impulsive impressions function in children and animals. The presence of the impulsive impression triggers a maintenance goal, and the maintenance goal gives this impression its impulsiveness. In the context of Stoic psychology, the maintenance goal has its basis in how nature constructs animals and children. The Stoics think that nature is provident and that nature constructs animals and children so that when they have certain impressions they naturally behave in ways conducive to their survival and well-being.

The Stoics think that when a child becomes an adult, although motivation still occurs in terms of impulsive impressions, “assent” (συγκατάθεσις) becomes necessary for impressions to issue in impulses. Assent is a function of reason. Because animals and children lack reason, they cannot assent. At the same time, they do not need to assent. In them, nature in its providence arranges things so that impulsive impressions automatically issue in impulses. Adults, however, according to the Stoics, act in terms of reason. They must assent to their impulsive impressions.

In the adult, the impulsiveness of an impulsive impression is a function of its relation to other states in the psychology. Only against the background of a belief (knowledge or opinion) about what is good or bad does assent to an impression constitutes an impulse. Animals and children naturally behave in accordance with the perfectly rational order of things in nature, but adults have reason. Their impulses are a function of their understanding of what the good is and how it applies the the circumstances. In this way, beliefs about what is good function like the maintenance goals that underwrite the impulsiveness of certain impressions in animals and children. If, for example, the adult believes it is good for him to find food and to eat it if he is hungry, then the impression "I am hungry" is impulsive for him. If he assents to this impression, then given the way nature constructs his psychology, his assent provides him with the impulse to find food and to eat it.

"The Stoics revert to Socrates' extreme intellectualism. They deny an irrational part of the soul. The soul is a mind or reason. Its contents are impressions or thoughts, to which the mind gives assent or prefers to give assent. In giving assent to an impression, we espouse a belief. Desires are just beliefs of a certain kind, the product of our assent to a so-called impulsive impression" (Michael Frede, "The Philosopher," 10).

Cognitive Impressions

Adults can assent on the basis of their beliefs (knowledge or opinion), but there is also another way for adults to assent to their impressions. Children and animals cannot have beliefs, according to the Stoics, and although adults can have beliefs, there is a point in the transition from child to adult at which no assent has been given and hence there are no beliefs. The Stoics explain assent in this situation in terms of the presence of a special kind of impression. They think that nature in its providence makes the soul of the adult so that the presence of this kind of impression automatically results in the assent. Impressions of this sort are "cognitive impressions" (φαντασίαι καταληπτικαί). The propositional content of a cognitive impression is true, but merely having a true propositional content is not enough to make an impression cognitive. A cognitive impression is an impression with a true propositional content that is formed in a certain distinctive way.

"The criterion of truth they declare to be the cognitive impression (καταληπτικὴν φαντασίαν)..." (Diogenes Laertius, Lives of the Philosophers VII.54).

Children do not have reason and so have the same kind of sense-impressions as animals, but these sense-impressions give rise to natural notions or what the Stoics call "preconceptions" (προλήψεις) of colors, shapes, and other simple perceptual features. Human beings naturally develop so that from these simple notions, and the more complex notions that arise naturally from them, they have cognitive impressions that allow them to have "cognition" (κατάληψις) of things. In this way, nature in its providence ensures that human beings can properly orient themselves in the world.

"When a man is born, the Stoics say, he has the commanding-part of his soul (τὸ ἡγεμονικὸν μέρος τῆς ψυχῆς) like a sheet of paper ready for writing upon. On this he inscribes each of his conceptions (ἐννοιῶν). The first method of inscription is through the senses. For by perceiving something, e.g., white, they have a memory of it when it has departed. And when many memories of a similar kind have occurred, we then say we have experience (ἐμπειρίαν). For the plurality of similar impressions is experience. Some conceptions arise naturally in the aforesaid ways and undesignatedly, others through our own instruction and attention. The later are called 'conceptions (ἔννοιαι)' only, the former are called 'preconceptions (προλήψεων)' as well" (Pseudo-Plutarch, Placita 4.11; LS 39 E).

The Stoics understand reason in terms of preconceptions and the basic truths about the world that these conceptions embody. (This understanding of reason shows itself in Aristotle's Theory of Induction and seems ultimately to have its roots in Plato's Theory of Recollection.) These preconceptions form the basis for the recognition of consequence and incompatibility and thus for the ability to make inferences. The idea is that given, say, the preconception or natural notion of a human being, the adult is in a position to infer various things. Because, for example, it is part of the notion of a human being that there is a relation of consequence between being human and being mortal, the adult can infer of the human beings he sees that they are mortal.

"Reason (λόγος), for which we are called rational (λογικοί), is said to be completed from our preconceptions (προλήψεων) during our first seven years" (Pseudo-Plutarch, Placita 4.11; LS 39 E).

"[The question is] precisely how reason thus conceived [as consisting of natural notions and the knowledge they embody] gives rise to reasoning and inference. Galen in various places attributes to reason as a basic feature the ability to recognise consequence (akolouthis) and incompatibility (mache). These notions play a prominent role in Stoic thought. And it is tempting to think that the idea that it is a characteristic of reason to recognise consequence an incompatibility is of Stoic origin. If we make this assumption, the following account suggests itself. To have the notion, say, of a human being is to see that a relation of consequence or implication obtains between being a human being and being mortal; it also is to see that a relation of incompatibility obtains between being a human being and being devoid of reason. Thus the ability to recognise consequence and incompatibility is part of what it is to have natural concepts or, for that matter, any kind of concepts. But by having concepts, and thus being able to recognise consequence and incompatibility, we also are in a position to reason to make inferences" (Michael Frede "The Stoic Conception of Reason," 55).

For the Stoics, the wisdom that constitutes virtue consists in the perfection of reason. "But what is more divine, I will not say in man only, but in all heaven and earth, than reason? And reason, when it is full grown and perfected, is rightly called wisdom" (Cicero, On the laws I.7; cf. 22). This perfection is first of all a matter of properly grasping the truths about the world embodied in preconceptions.

The two-clause definition of a cognitive impression

"[A]ccording to the Stoics the cognitive impression (καταληπτικὴ φαντασία) is judged to be cognitive by the fact that it proceeds from an existing object and in such a way as to bear the impress and stamp of that existing object; and the existing object is approved as existent because of its exciting a cognitive impression" (Sextus Empiricus, Against the Ethicists 183).

"An impression (φαντασίαν) is an imprint on the soul: the name having been appropriately borrowed from the imprint made by the seal upon the wax. Of impressions, some are cognitive (καταληπτικήν) and some are not-cognitive (ἀκατάληπτον). The former, which they say is the criterion (κριτήριον) of reality, is defined as that which proceeds from a real object, agrees with that object itself, and has been imprinted seal-fashion and stamped upon the mind: the latter, or non-cognitive, that which does not proceed from any real object, or, if it does, fails to agree with the reality itself, not being clear or distinct (μὴ τρανῆ μηδὲ ἔκτυπον)" (Diogenes Laertius, Lives of Eminent Philosophers VII.1.45-46).

According to the Stoics, nature in its providence arranges things so that human beings can assent to their impressions so that knowledge is possible. Cognitive impressions are part of this arrangement. Human beings and things in the world are such that in the course of their ordinary interactions with the world human beings can have impressions that are cognitive. They must form their perceptions under the appropriate conditions. They cannot form the perception when they are sick or at other times in which their minds are not working normally, and they must perceive the objects under the conditions that are appropriate for the sensory organs. So, e.g., to determine by looking whether something is a human being, there must sufficient light to make the determination. In this situation, human beings have impressions that have a certain character. They are cognitive. This character is an objective feature of the impression. The impression "bear[s] the impress and stamp" of reality in a certain way. Cognitive impressions have a certain "clarity" and "distinctness," and nature in its providence has arranged things so that the propositional content of any impression with this character is true.

It is not straightforward to explain this "clarity" and "distinctness," but the suggestion is that as part of how nature in its providence arranges things so that human beings can orient themselves in the world, the objects human beings need to know about to live good lives have a set of features that makes them distinct from all other objects. In a cognitive impression, these features are represented with a "clarity" that results in assent and thus in a true belief. If, e.g., the impression of something that "this is F" is a cognitive impression, then the impression represents the distinctive features of the subject of the propositional content of the impression with a "clarity" that causes the human being to assent to the impression and thus the form the true belief that the subject is F.

"[A cognitive impression] being plainly evident (ἐναργὴς) and striking, lays hold of us, almost by the very hair, as they say, and drags us off to assent (συγκατάθεσιν), needing nothing else to help it to be thus impressive or to suggest its superiority over all others. For this reason, too, every man, when he is anxious to apprehend (καταλαμβάνεσθαι) any object exactly, appears of himself to pursue after an impression of this kind—as, for instance, in the case of visible things, when he receives a dim impression of the real object. For he intensifies his gaze and draws close to the object of sight so as not to go wholly astray, and rubs his eyes and in general uses every means until he can receive a clear and striking impression of the thing under inspection, as though he considered that the credibility of the apprehension (τὴν τῆς καταλήψεως πίστιν) depended upon that" (Sextus Empiricus, Against the Logicians I.257-258).

"[T]he distinctive mark of cognitive impressions is a causal feature in that it makes the mind react in a distinctive way and it is in this sense that the mind can discriminate cognitive and noncognitive impressions. ... But this is not to say that we cannot be aware of the fact that an impression is cognitive or noncognitive, that we cannot learn to tell whether an impression is clear and distinct or obscure and confused. In fact, the Stoic view seems to be that this is a matter of practice and that in principle one can get so good at it that one will never take a noncognitive impression to be cognitive" (Michael Frede, "Stoics and Skeptics on Clear and Distinct Impressions" in Essays on Ancient Philosophy, 168, 169).

Cognition, Knowledge, and Opinion

Cognitive impressions make knowledge possible because they provides the basis for the kind of assent required for knowledge. Assent to a cognitive impression issues the attitude that the Stoics call "cognition" (κατάληψις). All cognition is true, but not all cognition is knowledge. The Stoics thought that knowledge requires a certain kind of assent. Knowledge is the propositional attitude formed in assent that no rational means can force one to withdraw. The Stoics thought that this was the lesson of Socrates. As the Stoics understood him, although he seems to have thought that knowledge was possible, he thought that no one has knowledge because no one can see his way through his questioning. He always forces his interlocutors to withdraw their assent.

According to the Stoics, only the wise have knowledge. Those of us who are not wise have "opinion" (δόξα).

"Zeno ascribed reliability not to all impressions, but only to those which manifest, in a certain particular way, those objects which make the impressions. Such an impression, when it is perceived in itself, he called graspable... for how else could you express καταληπτόν. But when it is already received and accepted, he called it a grasp, resembling objects gripped in the hand, and in fact he had derived the actual term from manual prehension, nobody before having used the word in such a sense..." (Cicero, Academica I.41). "Zeno would spread out the fingers of one hand and display its open palm, saying 'An impression is like this.' Next he clenched his fingers a little and said, 'Assent is like this.' Then, pressing his fingers quite together, he made a fist, and said that this was cognition (and from this illustration he gave that mental state the name of κατάληψιν, which it had not before). Then he brought his left hand against his right fist and gripped it tightly and forcefully, and said that knowledge was like this and possessed by none except the wise man-but who is a wise man or ever has been even they themselves do not usually say" (Cicero, Academica II.145).

"Knowledge (ἐπιστήμην) they [the Stoics] say is steadfast cognition (κατάληψιν ἀσφαλῆ) or a state which in reception of impressions cannot be shaken by argument. Without the study of dialectic, they say, the wise man cannot guard himself in argument so as never to fall" (Diogenes Laertius, Lives of the Philosophers VII.47).

Skepticism about Knowledge

The Academic Skeptics tried to show the Stoics that they were committed to the view that they should withhold assent.

The Stoics thought that knowledge, although possible, is difficult to acquire. In this, they took themselves to follow Socrates. His questioning seemed to show that no one had knowledge, but rather than give up, he continued to search for answers. Whether and why Socrates thought that knowledge was possible is not clear, but the thought Stoics that knowledge is possible because nature, in its providence, arranges things so that human beings receive cognitive impressions in the ordinary course of their lives. The existence of cognitive impressions makes it possible for human beings to acquire knowledge. Human beings do not have the ability to distinguish true from false impressions in all cases, but they can distinguish cognitive impressions from other impressions.

The Academic Skeptics also took themselves to be following Socrates. They thought Socrates showed that no human being could have knowledge, they tried to use the sort of questioning Socrates uses to get the Stoics to agree that for every true impression, it is possible that there is another just like it that is false. It follows, in this case, that there are no cognitive impressions.

"[The] argument [against the Stoics] is constructed as follows: 'Some impressions are true, others false; and what is false cannot be perceived. But a true impression is invariably of such a sort that a false impression also could be of exactly the same sort; and among presentations of such a sort that there is no difference between them, it cannot occur that some are capable of being perceived and others are not. Therefore there is no impression that is capable of being perceived" (Cicero, Academica II.40-41).

(The English word 'perception' derives from Cicero's use of perceptio for κατάληψις ("cognition" or, literally, "grasp"). The Latin perceptio means "a taking, receiving; a gathering in, collecting." So although Cicero sometimes uses perceptio for κατάληψις, he does not mean to suggest that the Stoics thought that cognitive impressions are the form sense impressions take in adults.)

In response to this questioning, the Stoics did not agree that for every true impression there could be another like it that is false. The reason is that for them indiscernibles are identical. (The identity of indiscernibles is the view that if x and y are indiscernible, then x is y. The identity of indiscernibles is not the same as indiscernibility of identicals. The indiscernibility of identicals is the view that if x is y, then x and y are indiscernible. The indiscernibility of identicals is commonly thought to be a truth of logic. The identity of indiscernibles is more controversial.)

The Stoic Theory of the Good Life

On the good life and happiness, the Stoics developed the view they thought Socrates held. The Stoics think that the good life for a human being is a matter of having virtue, that having virtue is having a certain wisdom, and that this wisdom includes knowledge of what is good and what is bad. They think that human beings acquire reason as they mature, that knowledge of what the good is is part of reason, and that the actions of adult are completely a matter of reason. In this way, the Stoics suppose that nature in its providence constructs human beings so that they get a set of cognitions adequate for understanding of certain aspects of the world in which they exist. They need this understanding to orient themselves properly and thus be in a position to live good lives.

The Stoics identify nature with fate and think of it as a perfectly rational agent that arranges thinks in the world. They think that nature arranges things so that human beings develop a preconception of the good, but that human beings almost always misapply this knowledge. The Stoics think that when human beings transform from children to adults and thereby acquire reason, they form false beliefs about what is good and what is bad. They come to think what human beings ordinarily think about what is good and what is bad--that health is good, sickness is bad, and so on.

By reflecting on their behavior in a certain way, human beings will realize that the good applies to nature and that they should "live in agreement with nature." The Stoic sage has come to understand that nature is perfectly rational, that human beings have a part to play in maintaining the perfect rationality in nature, and that they play this part by acting rationally. He understands that the good does not apply to the world insofar as he eats when he is hungry, regains his health when he is sick, and so on, but instead applies to the world because nature is perfectly rationally. He does not know what will happen in every situation. The Stoic sage is not omniscient, but he knows that the order in nature is in general one in which human beings eat when they are hungry, recover when they are ill, and so on. This sets him apart from the Stoic fool whose impulses are excessive because he (like human beings generally) thinks that it is bad not to find food when he is hungry, not to regain his health when he is ill, and so on. The Stoic sage, because he "lives in agreement with nature," has a life that is good and is filled with a joy that vastly surpasses the pleasure in any other life.

In this way, the Stoics return to a more Socratic conception of the good life. The good life for the Stoics is not a life of "contemplation" (θεωρία), as it is for Plato and Aristotle. The Stoic sage does the same sort of things in his life that human beings do in life ordinarily. The difference is in his motivation. He is not confused about what is good and what is bad. He does not attribute a value to health, life, and other ordinary "goods" that they do not possess. It is reasonable for him, given his partial understanding of the order in nature, to try to arrange things so that he is healthy and maintains his life, but he is not upset despite his precautions if he falls ill and does not recover. He realizes that his not recovering in these circumstances is part of the perfectly rational order in nature.

"Zeno was the first (in his treatise On the Nature of Man) to designate as the end 'life in agreement (ὁμολογουμένως) with nature' (or 'living agreeably to nature'), which is the same as a life according to virtue, virtue being the goal towards which nature guides us. ... And this is why the end may be defined as life in accordance with nature, or, in other words, in accordance with our own human nature as well as that of the universe, a life in which we refrain from every action forbidden by the law common to all things, that is to say, the right reason which pervades all things, and is identical with this Zeus, lord and ruler of all that is. And this very thing constitutes the virtue of the happy man and the smooth current of life, (τὴν τοῦ εὐδαίμονος ἀρετὴν καὶ εὔροιαν βίου) when all actions promote the harmony of the destiny (δαίμονος) dwelling in the individual man with the will (βούλησιν) of him who orders the universe" (Diogenes Laertius, Lives of the Philosophers VII.87-88).

"From the gods something divine (δαιμόνιον) comes to me. ... I have had this from my childhood; it is a sort of voice that comes to me, and when it comes it always holds me back from what I am thinking of doing, but never urges me forward" (Apology 31c-d). "[In my life before this trial] my usual prophetic divine thing (ἡ γὰρ εἰωθυῖά μοι μαντικὴ ἡ τοῦ δαιμονίου) always spoke to me very frequently and opposed me even in very small matters, if I was going to do anything I should not; but now, as you yourselves see, this thing which might be thought, and is generally considered, the greatest of evils has come upon me; but the divine sign (τὸ τοῦ θεοῦ σημεῖον) did not oppose me either when I left my home in the morning, or when I came here to the court, or at any point of my speech, when I was going to say anything; and yet on other occasions it stopped me at many points in the midst of a speech; but now, in this affair, it has not opposed me in anything I was doing or saying. What then do I suppose is the reason? I will tell you. This which has happened to me is doubtless a good thing, and those of us who think death is an evil must be mistaken" (Apology 39e-40c; cf. Timaeus 90a).

Perseus Digital Library: Diogenes Laertius, Lives of Eminent Philosophers,
Henry George Liddell, Robert Scott, A Greek-English Lexicon: ἀπάθεια, δαιμόνιον, ἐναργής , ἔννοια, εὐπάθεια, κατάληψις, πρόληψις , συγκατάθεσις
Charlton T. Lewis, Charles Short, A Latin Dictionary: cognitio, comprehensio, perceptio, percipio, perturbatio, ratio, scientia, voluntarius

Arizona State University Library. Loeb Classical Library: Cicero, On Ends, On the Nature of the Gods. Academics