Through memory and consciousness we reach that function of the mind called the intellect or reason, the product of which is _thought_. Its physical accompaniments are chemical action, and an increase of temperature in the brain. But the sum of the physical forces thus evolved is not the measure of the results of intellectual action. These differ from other forms of force in being incommensurate with extension. They cannot be appraised in units of quantity, but in quality only. The chemico-vital forces by which a thought rises into consciousness bear not the slightest relation to the value of the thought itself. It is here as in those ancient myths where an earthly maiden brings forth a god. The power of the thought is dependent on another test than physical force, to wit, its _truth_. This is measured by its conformity to the laws of right reasoning, laws clearly ascertained, which are the common basis of all science, and to which it is the special province of the science of logic to give formal expression. Physical force itself, in whatever form it appears, is only known to us as feeling or as thought; these alone we know to be real; all else is at least less real.[18-1] Not only is this true of the external world, but also of that assumed something, the reason, the soul, the ego, or the intellect. For the sake of convenience these words may be used; but it is well to know that this introduction of something that thinks, back of thought itself, is a mere figure of speech. We say, “_I_ think,” as if the “I” was something else than the thinking. At most, it is but the relation of the thoughts. Pushed further, it becomes the limitation of thought by sensation, the higher by the lower. The Cartesian maxim, _cogito ergo sum_, has perpetuated this error, and the modern philosophy of the _ego_ and _non-ego_ has prevented its detection. A false reading of self-consciousness led to this assumption of “a thinking mind.” Our personality is but the perception of the solidarity of our thoughts and feelings; it is itself a thought. These three manifestations of mind--sensations, emotions and thoughts--are mutually exclusive in their tendencies. The patient forgets the fear of the result in the pain of the operation; in intense thought the pulse falls, the senses do not respond, emotions and action are absent. We may say that ideally the unimpeded exercise of the intellect forbids either sensation or emotion.