According to Aristotle, [a thesis] is a paradoxical supposition of one of the well-known [thinkers] in the domain of philosophy,[1] that is, a problem. However, not every problem is a thesis,[2] but [a thesis is] the supposition contrary to the common opinions of one of the well-known [thinkers] in the domain of philosophy. This is why also the champion of the opinion, in constituting his thesis, must apply himself to a problem of the following sort: for example, whether or not all things come together and are always in process of generation, but nothing is ever [stable], according to Heraclitus;[3] and whether or not what exists is unitary and motionless, as Parmenides thinks;[4] and whether or not there is motion, as Zeno thinks;[5] and whether or not health [is] a good, as Chrysippus says;[6] and whether or not it is [possible] to contradict, as Antisthenes thinks.[7]
What is by convention (thesis) is distinguished from what is by nature (phusis), as occurs in the case of the convention of names. In fact we also say there that names [exist] by convention, meaning not by nature. Some people already call 'theses' also rhetorical problems -- to which it is more usual to apply the name of 'hypothesis' -- because they believe that such [names] are constituted in certain underlying and defined things. Aristotle also calls theses 'hypotheses'.