What is your epistemology?

[Existentialist]

And you just redefined "They are well defined concepts" to be "Concepts have no definition".
Congratulations, you are a complete and utter waste of space (and by space i mean caramel, since space doesn't actually have a definition, right?)

I think you are taking my position to a logical extreme which would only be valid in a universe where all partially stated positions are automatically taken to a logical extreme. Useful as an insult but hardly constructive in a conversation. The point I was making when I said we all invent our own versions was, we all have to invent concepts to try and form an understanding that coincides with the concepts we are introduced to, but we can never know that our understanding truly coincides with the understanding of others. In this sense our understanding of concepts can only be subjective. As Adrian rightly pointed out,

It doesn't matter how likely something is of being true; if there is an alternative explanation, however bizarre or unsettling that might be, we cannot say that "gravity is known truth". Whether through the Last Thursday example, or through delusions of our own minds, I believe it is impossible to "know" these things in any absolute way. As such, they remain very justified beliefs, but beliefs nonetheless.

Sometimes, we may freely admit that we might as well not take issue with an external, 'objective' explanation of a concept, because we need to get through the day. My invention of the concept of a banana and my belief about what a banana is seems to fit quite nicely with the curved yellow fruit in the supermarket, that suits me. At other times we may choose to insist that the ideas we have invented do not and cannot coincide with, for example, a broadly accepted idea such as "the deficit must be eliminated". The same principle of choice applies to every single concept we come across. I agree, there is huge scope for fraud and insincerity in this position, but there is also huge scope for authenticity and unconstrained freedom to discover new truths.

PS I agree, my grip on reality is long gone, and I'm distinctly proud of it! Whenever I type a 'u' on my keyboard, my screen produces a 'g'. I love being weird.