The physics of recent years has made heavy demands upon our capacity for realization. The electron theory, with its analysis of matter into “disembodied charges of electricity” required, for its understanding, the breaking up of old habits of thought. To young students the idea was, at first, extremely baffling――almost nonsense. To realize it one had to make more abstract one’s idea of matter until the notion of “substance” was replaced by the notion of “behaviour”. Anything that behaved in the way characteristic of matter was matter. The central idea of the restricted principle of relativity, the idea of different time-systems, was still more difficult to grasp. In this case we had to become convinced that our ordinary idea of simultaneity, an idea which seemed perfectly clear, was really a bogus idea. The attacks on the theory of relativity show, for the most part, merely that their authors are unable to abandon old habits of thought. With the complete theory of relativity, as we have it now, the task of adjustment has become enormous. There cannot be, even now, more than very few scientific men who naturally approach a problem from the point of view of relativity theory. In most cases a conscious effort of mental preparation is required, such as occurs when a novelist, sitting down to continue his work, deliberately thinks himself into the appropriate frame of mind. Yet doubtless the next generation or so will think in terms of relativity theory as naturally as we thought in terms of the Newtonian system. I would not hold it as impossible that the human mind may come to realize, imaginatively as well as logically, the four-dimensional space-time continuum. But it seems that the mind of the physicist, at any rate, will have to do more than become familiar with relativity theory. It will have to accommodate itself somehow to the quantum theory for, although we can write down the laws which govern sub-atomic phenomena and make deductions from them, these laws are, at present, unintelligible. An electron behaves as if it had foreknowledge of what it was about to do and could make the mathematical calculations necessary to achieve its end. We cannot admit this to be possible, and we can only suppose that the difficulty arises from the way we think about things. We must learn to think in a different way, and what the consequences of that new way of thinking will be no one can say. We know very little of the possibilities of the development of the human consciousness.