In chess or in contract bridge there are rules defining the pieces or deck of cards, and then rules and procedures for correctly using them. But in logic, the first part is entirely missing, and you end up with rules and procedures without really defining the objects to be manipulated. In chess, on the other hand, the variables are strictly limited to a specific set of objects.

If what you say and imply is true, then doesn't the philosophical therefore of your own argument being meaningless?

If one concludes that no conclusion is inevitable, then doesn't that conclusion itself also fall short of being inevitable?

Inevitability in chess only becomes possible by first accepting certain premises. Can't the situation in logic be exactly the same?

In the end, I think we reach the point of recognizing a Cartesian skepticism... The conclusion that one's own consciousness exists would then be the only philosophical therefore that can "mean" anything.

So what do you think? Am I hot, cold... lukewarm? Did I get the point but make a mistake in my reasoning or just say something that you disgaree with?

Yes, but I'm just making conversation. I'm not rigorously trying to put forward an airtight logical argument.

But it's NOT exactly the same. When is a a set of ideas so closed that it cannot be bent with different assumptions or interpretations?

I don't necessarily accept Descartes' exercise or his conclusion, so I'm not sure that skepticism leads us inevitably there. But that's not really the point anyway -- I'm talking about how logical argumentation is inevitably going to be in a widely open system, and thus no logically-derived conclusion can ever have the same inevitability as the solution to a Sudoku puzzle.

I think we're having the same conversation, more or less, and I agree with your point that yes it's all about a priori assumptions and rules. That said, human ideas can NEVER have sufficient clarity and sufficiently foolproof rules that logical conclusions will be airtight; and there ARE logical deductions we can make (like what's on the other side of the coin) that are completely airtight.

[1]unlike these games and puzzles, human logic when used to make reasoned arguments is NOT a closed system. Part of this is the lack of complete correspondence between a thought and a way of expressing it with no ambiguity.
[2]This makes me think that any appeal to logic or reason in philosophy must of necessity acknowledge their fallibility, and therein any derived conclusions.
[3]If even mathematics cannot be a pure tautology, as Godel taught us, then reason certainly cannot. So how can we be sure that any philosophical therefore really means anything?

Logic can help determine if statements and arguments are valid, but cannot determine if the premises and statements are true. Under most circumstances, if one can establish their truth, then the conclusion if validly drawn would be true. The hard part is to present something like a substantial argument for the truth of the premises; much of philosophy has been just such attempts.

Take an utterly simple logical puzzle. You have a two sided coin; on one side is heads and the other side is tails. You flip the coin and it lands with the head facing up -- THEREFORE the other side is tails. There is nothing at all that compromises the inevitability of this