In this their fourth conversation the 17th century philosopher, John Locke and the 21st century linguist, Terence Moore, consider a question not fully answered even today: what might count as the key distinction beween man and animals, or in Locke's phrase what ‘puts a perfect distinction between Man and Brutes.’ In the Essay Concerning Human Understanding, Locke considers two possible linguistic candidates: the ability to use language appropriately, and the ability to ‘quit Particulars’. As Locke and Moore explore these possibilities they come to see that the distinction between man and animals is not as clear-cut as previous generations have believed. Locke tentatively posits a third possible distinction based on a central idea in his Essay – one however Moore is compelled to dismiss, though he, in return, also tentatively, offers a fourth.