1138b We have already said¹ that it is right to choose the mean and to avoid excess and deficiency, and that the mean is prescribed by the right principle. Let us now analyze the latter notion. In the case of each of the moral qualities or dispositions that have been discussed, as with all the other virtues also, there is a certain mark to aim at, on which the man who knows the principle involved fixes his gaze, and increases or relaxes the tension² accordingly; there is a certain standard determining those modes of observing the mean which we define as lying between excess and defect, being in conformity with the right principle. This bare statement however, although true, is not at all enlightening. In all departments of human endeavor that have been reduced to a science, it is true to say that effort ought to be exerted and relaxed neither too much nor too little, but to the medium amount, and as the right principle decides. Yet a person knowing this truth will be no wiser than before: for example, he will not know what medicines to take merely from being told to take everything that medical science or a medical expert would prescribe. Hence with respect to the qualities of the soul also, it is not enough merely to have established the truth of the above formula; we also have to define exactly what the right principle is, and what is the standard that determines it.³ Now we have divided the Virtues of the Soul into two groups, 1139athe Virtues of the Character and the Virtues of the Intellect. The former, the Moral Virtues, we have already discussed. Our account of the latter must be prefaced by some remarks about psychology. It has been said before⁴ that the soul has two parts, one rational and the other irrational. Let us now similarly divide the rational part, and let it be assumed that there are two rational faculties, one whereby we contemplate those things whose first principles are invariable, and one whereby we contemplate those things which admit of variation: since, on the assumption that knowledge is based on a likeness or affinity of some sort between subject and object, the parts of the soul adapted to the cognition of objects that are of different kinds must themselves differ in kind. These two rational faculties may be designated the Scientific Faculty and the Calculative Faculty respectively; since calculation is the same as deliberation, and deliberation is never exercised about things that are invariable, so that the Calculative Faculty is a separate part of the rational half of the soul. We have therefore to ascertain what disposition of each of these faculties is the best, for that will be the special virtue of each. But the virtue of a faculty is related to the special function which that faculty performs.

Now there are three elements in the soul which control action and the attainment of truth: namely, Sensation, Intellect,⁵ and Desire. Of these, Sensation never originates action, as is shown by the fact that animals have sensation but are not capable of action.^{6 7}Pursuit and avoidance in the sphere of Desire correspond to affirmation and denial in the sphere of the Intellect. Hence inasmuch as moral virtue is a disposition of the mind in regard to choice,⁸ and choice is deliberate desire,⁹ it follows that, if the choice is to be good, both the principle must be true and the desire right, and that desire must pursue the same things as principle affirms. We are here speaking of practical thinking, and of the attainment of truth in regard to action; with speculative thought, which is not concerned with action or production, right and wrong functioning consist in the attainment of truth and falsehood respectively. The attainment of truth is indeed the function of every part of the intellect, but that of the practical intelligence is the attainment of truth corresponding to right desire.¹⁰ Now the cause of action (the efficient, not the final cause) is choice,¹¹ and the cause of choice is desire and reasoning directed to some end. Hence choice necessarily involves both intellect or thought and a certain disposition of character [¹² for doing well and the reverse in the sphere of action necessarily involve thought and character]. Thought by itself however moves nothing, but only thought directed to an end, and dealing with action. 1139bThis indeed is the moving cause of productive activity¹³ also, since he who makes some thing always has some further end in view: the act of making is not an end in itself, it is only a means, and belongs to something else. Whereas a thing done is an end in itself: since doing well (welfare¹⁴) is the End, and it is at this that desire aims. Hence Choice may be called either thought related to desire or desire related to thought; and man, as an originator of action, is a union of desire and intellect. (Choice is not concerned with what has happened already: for example, no one chooses to have sacked Troy; for neither does one deliberate about what has happened in the past, but about what still lies in the future and may happen or not; what has happened cannot be made not to have happened. Hence Agathon is right in saying This only is denied even to God, The power to make what has been done undone.) The attainment of truth is then the function of both the intellectual parts of the soul. Therefore their respective virtues are those dispositions which will best qualify them to attain truth.

Let us then discuss these virtues afresh, going more deeply into the matter. Let it be assumed that there are five qualities through which the mind achieves truth in affirmation or denial, namely Art or technical skill,¹⁵ Scientific Knowledge, Prudence, Wisdom, and Intelligence. Conception and Opinion are capable of error. The nature of Scientific Knowledge (employing the term in its exact sense and disregarding its analogous uses) may be made clear as follows. We all conceive that a thing which we know scientifically cannot vary; when a thing that can vary is beyond the range of our observation, we do not know whether it exists or not. An object of Scientific Knowledge, therefore, exists of necessity. It is therefore eternal, for everything existing of absolute necessity is eternal; and what is eternal does not come into existence or perish. Again, it is held that all Scientific Knowledge can be communicated by teaching, and that what is scientifically known must be learnt. But all teaching starts from facts previously known, as we state in the Analytics,¹⁶ since it proceeds either by way of induction, or else by way of deduction. Now induction supplies a first principle or universal, deduction works from universals; therefore there are first principles from which deduction starts, which cannot be proved by deduction; therefore they are reached by induction. Scientific Knowledge, therefore, is the quality whereby we demonstrate,¹⁷ with the further qualifications included in our definition of it in the Analytics,¹⁸ namely, that a man knows a thing scientifically when he possesses a conviction arrived at in a certain way, and when the first principles on which that conviction rests are known to him with certainty—for unless he is more certain of his first principles than of the conclusion drawn from them he will only possess the knowledge in question accidentally.¹⁹ Let this stand as our definition of Scientific Knowledge.

1140aThe class of things that admit of variation includes both things made and actions done. But making is different from doing (a distinction we may accept from extraneous discourses²⁰). Hence the rational quality concerned with doing is different from the rational quality concerned with making; nor is one of them a part of the other, for doing is not a form of making, nor making a form of doing. Now architectural skill, for instance, is an art, and it is also a rational quality concerned with making; nor is there any art which is not a rational quality concerned with making, nor any such quality which is not an art. It follows that an art is the same thing as a rational quality, concerned with making, that reasons truly. All Art deals with bringing some thing into existence; and to pursue an art means to study how to bring into existence a thing which may either exist or not, and the efficient cause of which lies in the maker and not in the thing made; for Art does not deal with things that exist or come into existence of necessity, or according to nature, since these have their efficient cause in themselves. But as doing and making are distinct, it follows that Art, being concerned with making, is not concerned with doing. And in a sense Art deals with the same objects as chance, as Agathon says: Chance is beloved of Art, and Art of Chance. Art, therefore, as has been said, is a rational quality, concerned with making, that reasons truly. Its opposite, Lack of Art, is a rational quality, concerned with making, that reasons falsely. Both deal with that which admits of variation.