I recently encountered a series of youtube videos which claimed to find errors in the logic and algebra underpinning Keynesian economics.

Excuse me for being sceptical, but I think its strange that it has taken 75 yrs to find these flaws. Surely if they existed, it would be well known by now? Below is a link to one of these videos. Has the author found a previously unknown flaw? Or is this well-known? Or has the author, in fact, got it completely wrong?

I said the guy I was talking to left "a" out of his consumption function.

The guy used Y. Without him labeling it, I don't know whether he meant total of disposable, and he would not show the whole equation so I could tell.

And substituting Y_{t} - T for Y_{d }is asinine.

1) You are substituting the equation back into itself. The only reason they do that is to get around the problem that they can't factor Y_{t} out of Y_{t} - bY_{d}.

2) You retroactively apply the marginal propensity to consume, b, to Y_{t} and T, and end up with -bT.

Here's an example:

If Fruit F, consists of Apples A, and Oranges O, and we subtract the Oranges, we are left with the Apples: F - O = A F = A + O Let's say the marginal propensity for apples to be red is b = 0.8, so 0.8 of apples are red, and (1-b) are not red: F = bA + (1-b)A + O then we make the asinine substitution of (F - O) for A: F = b(F - O) + (1-b)(F - O) + O and multiply out: F = bF - bO + (1-b)F -(1-b) O + O We have now erroneously, retroactively, applied the marginal propensity for apples to be red to oranges and all fruit, and find that 0.8 of oranges are red, and 0.8 of the fruit is red.

Keynesians go further and disguise the (1-b)Y_{d} so the scam is not evident.

We have now erroneously, retroactively, applied the marginal propensity for apples to be red to oranges and all fruit, and find that 0.8 of oranges are red, and 0.8 of the fruit is red.

Not at all. Nowhere does it say that 0.7 of oranges are red. They're just numbers for calculation. The equation is correct.

ok, now you are just trolling.
This is just a friendly warning, which I may regret extending, but the important point is, behave and we will hear you out.

Where there is no property there is no justice; a proposition as certain as any demonstration in Euclid

Fools! not to see that what they madly desire would be a calamity to them as no hands but their own could bring

Tugwit, the problem here is that you don't want to take into account the objections that have been made and that you didn't bother to respond just because "this sounds keynesian".

NirgrahamUK, Wheylous, and Chris S. If you disagree with them, you have to show "where" exactly they are wrong. Everyone is waiting. (and don't tell me "I have already replied")

Tugwit, can you answer their objections without dismissing them as Keynesian nonsense? I'm not saying that they're right, I'd just like to hear the fullness of your responses, from start to finish.