The most fundamental principle which pervades every one of the sciences, and agrees with the actions of every natural form of energy without exception, including mind, is, that of consistency or non-contradiction. No machine or scientific apparatus of any kind can perform two contradictory acts at the same time. It is both a physiological and psychological fact, that we cannot experience two contradictory sensations, nor perceive two contradictory ideas at the {100} same instant. We can neither feel, perceive, nor observe, one thing, whilst we are feeling, perceiving, or observing, one of a contradictory nature; nor can we perform any two contradictory acts of comparison, inference, imagination, or volition, simultaneously. As also two mental actions are often not exactly alike, or entirely harmonious, they must so far as they are really contradictory, be mutually exclusive; and one of them must partly prevent the other, the strongest one prevailing, and this general truth is commonly though not explicitly, recognised in the maxim, that to do anything well, we must do only one thing at a time. In accordance with the universal truth, that contradictions cannot co-exist, it is well-known that one disease frequently expels another from our frame, and the action of counter-irritants is based upon the same principle. The fortitude of martyrs may probably be explained by this power of one set of ideas and feelings to exclude another, and the facts of mental physiology afford plenty of other examples. It is probably because we cannot simultaneously perform two contradictory actions, that we cannot contemplate consciousness, or think of an idea and at the same time think of that act of thought. In accordance with this, even Newton, and other great geniuses, have been unable to accurately describe the mental processes by means of which they arrived at their most difficult results. In consequence also of this, we cannot define consciousness, and are often {101} unable to directly observe or analyse our mental actions, especially those of a very abstruse or complex kind. Much of the knowledge of the operations of our mind, we are therefore obliged to obtain by indirect means; by analogies, and inferences from the phenomena of nature, &c., and in this way our knowledge of mental action largely depends upon our acquaintance with physical and chemical science, and can only advance as it advances. To clearly understand one subject we are often obliged to study several others. Ignorance of science in general, and of cerebral physiology in particular, is the chief obstacle to our acquiring a more accurate knowledge of mind.