The deep pleasure with which I received the invitation to deliver this lecture was tinged with dismay, for as a physiologist I am only too frequently reminded that the contributions which psychiatry has received from physiology are, in fact, almost negligible. I was therefore bound to ask myself very seriously whether I could honestly accept the honour without committing myself to stray far beyond the confines of my own subject. Finally, I satisfied my conscience by deciding that the invitation was issued in the spirit of hope rather than satisfaction, and I trust that I shall not be misinterpreting the intentions of this Royal and learned body if I feel encouraged to develop a speculative rather than a purely empiric theme. My aim is to outline the manner in which the study of brain physiology may conceivably enable us to define the physical parameters of mental experience, and the title which I first suggested for the lecture was “The Frame of Reason.” Vivid though the image may be for me, I admit that the more laconic title is better, for it does at least give us a landmark and a limit to our exploring.