You will see I said in my post that low level drug taking was common. For that very reason a smoking caterpillar is no evidence that Lewis Carroll wrote under the influence of drugs.

I have not read that much about it, to be honest, but to the best of my knowledge no-one made a link between Carroll and drugs until the 60s, which is a great exemplar of confusing correlation and causation: "I see weird stuff when high; Carroll wrote weird stuff; he must have been on drugs". That is one of the invalid forms of logic known since the Greeks and as a teacher of logic he would have given it short shrift.

As you say, it is difficult to prove one way or the other. But the circumstantial evidence like that book of logic (have you read it yet?) shows he is at least capable of writing highly imaginative stuff while not high.

However, after all that, you did not start with "it is difficult to prove one way or the other". You started with the assertion he was on drugs. That is what I am picking you up in.