When you posit something, you put forward an idea or assumption as a basis for reasoning or discussion. Scientists posit theories they want to test. Philosophers posit fundamental principles about how the world works. You're not necessarily claiming something is absolutely true: you're proposing it as a starting point to see where the thinking leads.

A teacher might posit that students learn better when they're interested in a subject, then design lessons to test that idea. A detective might posit that the suspect had an accomplice, then look for evidence to support or disprove it. When you posit something, you're essentially saying, “Let's assume this is true and see what follows.”