If we can doubt logic then philosophical enquiry is meaningless?

If logic itself is simply definition, then scepticism leads us to doubt if it can give us any truth about the world. Philosophers regularly doubt sense perception, time and even the self and then use logical reasoning to build a view of the world around such problems...but ALL of this is based on the assumption that logic is certain. If this can be doubted then how can we trust anything that any philosopher has ever said? Good philosophers try not to make assumptions. Is logic one that we are simply forced to make?