Stephen Hawking, again

In the introduction of Stephen Hawking's book A Brief History of Time, he writes this:

Apart from being unlucky enough to get ALS, or motor neuron disease, I have been fortunate in almost every other respect...[Many people] have made it possible for me to lead a fairly normal life and to have a successful career.  I was again fortunate in that I chose theoretical physics, because that is all in the mind.  So my disability has not been a serious handicap.