Aristotle has just divided names into ten categories in the Categories. Now he explains the relationship between combining concepts and truth.
None of the above is said just by itself in any affirmation, but by the combination of these with one another an affirmation is produced. For every affirmation, it seems, is either true or false- but of things said without any combination none is either true or false (e.g. man, white, runs, wins).
Names by themselves do not affirm anything. When two or more names are combined, the very act of combining them affirms that they are combined in reality. Every affirmation is either true or false, but without combination, nothing is true nor false.
This sets up Aristotle’s version of the principle of bivalence. Every combination of concepts is either true or false. This is much different than the modern principle of bivalence. For example, suppose we combined is a number with unicorn. We get Unicorn is a number. That is false. According to the modern principle, that would make Unicorn is not a number true. But Aristotle does not have to agree. He could claim that Unicorn is not a number is false too. Both statements are false because unicorns don’t exist. So Aristotles principle is different from the current principle in logic.
This also answers an earlier question. Whenever we speak of subject and predicate, truth comes into the picture because we are affirming something. But simply using a name is not making an affirmation. Therefore the four categories have to do with truth but the ten categories have to do with reference.
Next, Aristotle discusses substance and distinguishes between primary and secondary substance.