The first two conditions have generally been the most challenged, while the third appears to be the "least" controversial. Some philosophers have claimed that there is, in fact, no problem in believing the content of Moore's sentences (e.g. David Rosenthal). Others (e.g. Sydney Shoemaker) hold that an explanation of the problem at the level of belief will automatically provide us with an explanation of the absurdity at the level of assertion via the linking principle that what can reasonably be asserted is determined by what can reasonably be believed. Some have also denied (e.g. Rosenthal) that a satisfactory explanation to the problem need be uniform in explaining both the omissive AND commissive versions. Most of the explanations offered of Moore's paradox are united in holding that contradiction is at the heart of the absurdity.