Seeing that of relatives some are of necessity found in, or about, the things in relation to which they happen at any time to be used (e.g. composition and state and balance; for in nothing else can the aforesaid terms possibly be found except in the things in relation to which they are used), while others need not be found in the things in relation to which they are used at any time, though they still may be (e.g. if the soul is an object of knowledge, for it is quite possible that the soul should possess knowledge of itself, but it is not necessary for knowledge