You have instructed us, most excellent Tatian, to forward for your use a discourse upon the soul, laying it out in effective demonstrations. And this you have asked us to do without making use of the testimonies of Scripture, -- a method which is opened to us, and which, to those who seek the pious mind, proves a manner of setting forth doctrine more convincing than any reasoning of man. You have said, however, that you desire this, not with a view to your own full assurance, taught as you already have been to hold by the Holy Scriptures and traditions, and to avoid being shaken in your convictions by any subtleties of man's disputations, but with a view to the confuting of men who have different sentiments, and who do not admit that such credit is to be given to the Scriptures, and who endeavour, by a kind of cleverness of speech, to gain over those who are unversed in such discussions. Wherefore we were led to comply readily with this commission of yours, not shrinking from the task on account of inexperience in this method of disputation, but taking encouragement from the knowledge of your good-will toward us. For your kind and friendly disposition towards us will make you understand how to put forward publicly whatever you may approve of as rightly expressed by us, and to pass by and conceal whatever statement of ours you may judge to come short of what is proper. Knowing this, therefore, I have betaken myself with all confidence to the exposition. And in my discourse I shall use a certain order and consecution, such as those who are very expert in these matters employ towards those who desire to investigate any subject intelligently.
First of all, then, I shall propose to inquire by what criterion the soul can, according to its nature, be apprehended; then by what means it can be proved to exist; thereafter, whether it is a substance or an accident; then consequently on these points, whether it is a body or is incorporeal; then, whether it is simple or compound; next, whether it is mortal or immortal; and finally, whether it is rational or irrational.
For these are the questions which are wont, above all, to be discussed, in any inquiry about the soul, as most important, and as best calculated to mark out its distinctive nature. And as demonstrations for the establishing of these matters of investigation, we shall employ those common modes of consideration by which the credibility of matters under hand is naturally attested. But for the purpose of brevity and utility, we shall at present make use only of those modes of argumentation which are most cogently demonstrative on the subject of our inquiry, in order that clear and intelligible notions may impart to us some readiness for meeting the gainsayers. With this, therefore, we shall commence our discussion.