Aristotle (384–322 BCE) was an ancient Greek philosopher, credited with conceiving ways of understanding knowledge which were the forerunners of modern science. Here, he explains the workings of reason.
Let it be assumed that the states by virtue of which the soul possesses truth … are five in number: art, scientific knowledge, practical wisdom, philosophic wisdom, intuitive reason …
Practical wisdom is the quality of mind concerned with things just and noble and good for man … Regarding practical wisdom … it is thought to be the mark of a man of practical wisdom to be able to deliberate well about what is good and expedient for himself, not in some particular respect, e.g. about what sorts of thing conduce to health or to strength, but about what sorts of thing conduce to the good life in general … [S]ome even of the lower animals have practical wisdom, namely those which are found to have a power of foresight with regard to their own life …
[W]hile no one is thought to be a philosopher by nature, people are thought to have by nature judgement, understanding, and intuitive reason … [A] particular age brings with it intuitive reason and judgement; this implies that nature is the cause … Therefore we ought to attend to the undemonstrated sayings and opinions of experienced and older people or of people of practical wisdom not less than to demonstrations; for because experience has given them an eye they see aright …
[P]hilosophic wisdom is scientific knowledge, combined with intuitive reason, of the things that are highest by nature … Scientific knowledge is … a state of capacity to demonstrate …
When one thing is predicated of another, all that which is predicable of the predicate will be predicable also of the subject. Thus, ‘man’ is predicated of the individual man; but ‘animal’ is predicated of ‘man’; it will, therefore, be predicable of the individual man also: for the individual man is both ‘man’ and ‘animal’.
… [R]easoning is an argument in which, certain things being laid down, something other than these necessarily comes about through them. (a) It is a ‘demonstration’, when the premisses from which the reasoning starts are true and primary, or are such that our knowledge of them has originally come through premisses which are primary and true: (b) reasoning, on the other hand, is ‘dialectical’, if it reasons from opinions that are generally accepted. Things are ‘true’ and ‘primary’ which are believed on the strength not of anything else but of themselves: for in regard to the first principles of science it is improper to ask any further for the why and wherefore of them; each of the first principles should command belief in and by itself. On the other hand, those opinions are ‘generally accepted’ which are accepted by every one or by the majority or by the philosophers—i.e. by all, or by the majority, or by the most notable and illustrious of them. Again (c), reasoning is ‘contentious’ if it starts from opinions that seem to be generally accepted, but are not really such, or again if it merely seems to reason from opinions that are or seem to be generally accepted. For not every opinion that seems to be generally accepted actually is generally accepted.
All instruction given or received by way of argument proceeds from pre-existent knowledge. This becomes evident upon a survey of all the species of such instruction. The mathematical sciences and all other speculative disciplines are acquired in this way, and so are the two forms of dialectical reasoning, syllogistic and inductive; for each of these latter make use of old knowledge to impart new, the syllogism assuming an audience that accepts its premisses, induction exhibiting the universal as implicit in the clearly known particular.
Aristotle. 350 BCE-b. Nicomachean Ethics. The Internet Classics Archive, MIT. Book 6: 3, 12, 5, 7, 11, 5, 3. http://classics.mit.edu/Aristotle/nicomachaen.html