The 'blank slate' of the title is the human mind at birth, a view held, often implicitly, by our modern society, which has been conditioned to accept this by religions, progressive educationists, and the left in general. Those who hold the opposing view, that much of our nature is inherited, are subjected to frequent and vicious personal attacks (see the reviews of this book).
Pinker, however, is made of stern stuff, and has put a large explosive device under his opponents with this book based, as it is, on carefully documented research and grounded in appropriate theory. He ranges from genetics to computational linguistics via neurology and statistical theory in dazzling fashion.
It might seem that the weight of evidence gathered might cause the book to be heavy going, but the writing is sharper, and the touch is lighter and more humorous than anyone has a right to expect. As an example, consider the following, after a discussion on the effects of ageing: "Forget 'As the twig is bent, so the tree grows', think 'Omigod, I'm turning into my parents'".
While there are parts to the book which some will question, Pinker has turned the searchlights of reason and common sense on much of the political correctness of our time, showing how ludicrous most of it is, and showing also how science is beginning to give us a better understanding of what is meant by 'human nature'. If 'the proper study of mankind is man' then this is the essential primer.