Self-criticism within an existing and coherent left, at least roughly unified around both goals and strategy, is of course essential. So-called "criticism" of THE LEFT (which doesn't exist) is not self-criticism but selfconscious withdrawal in amused superiority to all those folk struggling in adverse circumstances to create a left in which self-criticism would be possible.
It seems to me that this discussion doesn't make any sense unless one has some idea of what 'the left' means. There is a striking similarity between the semantics of this term and that of 'the party'. When Marx and Engels wrote the manifesto they used the term 'communist party' in a way that is quite different to the post-Lenin sense - it simply referred to the communists, not to some quasi-military organisation with its internal discipline, party 'line', etc.
The question with 'the left' is similar. Does it simply mean all those whose views are somehow 'left', or is it an organisational category? If it is the former, I suspect that there would be many who think of themselves as part of an existing left, but that if it means the latter, then I think there would be many like me who would want no part of it and who would not mind one bit that it doesn't exist. Why not?
This question of the left cannot be entirely separated from its history. The only thing I think that it has meant in the second sense that I mentioned above, i.e. since the second international, is a class alliance. A class alliance can take many forms: popular front, united front, entryism, etc. Both the second and third internationals promoted a notion of the left that involved the participation of communists in the parliamentary apparatus, usually in collaboration with a social democratic party. This was the basis of Lenin's castigating the 'ultra-left', i.e. for not wanting to do this. This of course has the effect of irreversibly splitting the left into two sections, the 'official' left and the 'extra-parliamentary' left.
So now the question is, which left has the more potential today, a first international type left, which was composed of communists and anarchists, or a second/third international type left which emphatically excluded 'leftwing communists' and actively encouraged collaboration with bourgeois parties?
Who are the original splitters?
Tahir