Reference The post that started it all

In order to clear up the original question I asked in a provocative manner, I have posed this question.

If you learn an algorithm from an open source project, is it OK to use that algorithm in a separate closed sourced project? And if not, does that imply that you cannot use that knowledge ever again? If you can use it, what circumstance could that be?

Just to clarify, I am not trying to evade a licence, otherwise I would not have asked the question in the first place. I believe this presents a difficult question and it is interesting to know where the debate can end up.