Lay has been used intransitively in the sense of "lie"

since the 14th century. The practice was unremarked until around 1770; attempts to correct it have been a fixture of schoolbooks ever since. Generations of teachers and critics have succeeded in taming most literary and learned writing, but intransitive lay persists in familiar speech and is a bit more common in general prose than one might suspect. Much of the problem lies in the confusing similarity of the principal parts of the two words. Another influence may be a folk belief that lie is for people and lay is for things. Some commentators are ready to abandon the distinction, suggesting that lay is on the rise socially. But if it does rise to respectability, it is sure to do so slowly: many people have invested effort in learning to keep lie and lay distinct. Remember that even though many people do use lay for lie, others will judge you unfavorably if you do.