Asklemmy

What is A in this case (that which is definitively known)? The fact one has a subjective experience of some kind of perceived reality and in this reality we see other people that tell us they're real and say they have separate minds to us? And then is B the proposition "reality is real to a degree insofar as other minds that appear to exist do indeed exist and are sentient and hold opinions"? Because I agree A doesn't necessarily entail B in that case, it is something that I'm assuming for sake of argument as a basis for further reasoning.

But if we presuppose both A and B are true (let's call them AB), then it seems like the information and observed, even anecdotal/firsthand experience we can obtain from reality (and especially if we trust secondhand sources, but even if not) appears to (uncertainly) create the grounds for a case to be made that, using reasoning and empirical observations (a combination of a priori and a posteriori), we can deduce that if a large number of people care about a wide variety of diverse interests (which seems to be deducible by AB + an average experience of life where you meet a significant number of people who aren't lying to you (=C)), then there is a high likelihood of at least one of them caring about a given subject or phenomenon?

So this is assuming some things, such as A (apparently known, so maybe not assumed), in addition to B & C, but if ABC, then is it really an invalid form of reasoning to conclude or speculate that D (someone cares) is likely? Is any form of argument which isn't entirely certain unequivocally invalid? Because then you can't really consider anything valid (aside from A, or things which are known beyond a shadow of doubt, even if you acknowledge their uncertainty), right? This is why certain elements of Descartes' philosophy seem absurd to me... in addition to the intuitively contradictory idea that "All that we can know is I think therefore I am, but also God exists and is an evil demon that has created a false reality(?)"...