Frege's recognition criterion for thoughts and its problems

According to Frege, we need a criterion for recognising when different sentences express the same thought to make progress in logic. He himself hedged his own equipollence criterion with a number of provisos. In the literature on Frege, little attention has been paid to the problems these provisos raise. In this paper, I will argue that Fregeans have ignored these provisos at their peril. For without these provisos, Frege's criterion yields wrong results; but with the provisos in place, it is of no use for Frege's purposes. This is connected to what Frege took to be the 'greatest difficulty for philosophy': natural language sentences don't just express thoughts; they convey evaluations and communicative hints. Because of this, Frege's recognition criterion for thoughts cannot be applied to them and we cannot make logical progress by 'recognising a thought in different linguistic guises'.