Book I contains a summary of Aristotle's method of investigation and a dialectical determination of the nature of the soul. He begins by conceding that attempting to define the soul is one of the most difficult questions in the world. But he proposes an ingenious method to tackle the question: just as we can come to know the properties and operations of something through scientific demonstration, i.e. a geometrical proof that a triangle has its interior angles equal to two right angles, since the principle of all scientific demonstration is the essence of the object, so too we can come to know the nature of a thing if we already know its properties and operations. It is like finding the middle term to a syllogism with a known conclusion. Therefore, we must seek out such operations of the soul to determine what kind of nature it has. From a consideration of the opinions of his predecessors, a soul, he concludes, will be that in virtue of which living things have life.