Thus much for the origin of the idea of cause. We have the same direct intuitive knowledge of cause that we have of effect; but we have not yet rendered a full and adequate account of the _principle of causality_. We have simply attained the notion of our personal causality, and we can not arbitrarily substitute our personal causality for all the causes of the universe, and erect our own experience as a law of the entire universe. We have, however, already seen (Chap. V.) that the belief in exterior causation is _necessary_ and _universal_. When a change takes place, when a new phenomenon presents itself to our senses, we can not avoid the conviction that it must have a cause. We can not even express in language the relations of phenomena in time and space, without speaking of causes. And there is not a rational being on the face of the globe--a child, a savage, or a philosopher--who does not instinctively and spontaneously affirm that every movement, every change, every new existence, _must_ have a cause. Now what account can philosophy render of this universal belief? One answer, and only one, is possible. The _reason_ of man (that power of which Comte takes no account) is in fixed and changeless relation to the principle of causation, just as _sense_ is in fixed and changeless relation to exterior phenomena, so that we can not know the external world, can not think or speak of phenomenal existence, except as _effects_. In the expressive and forcible language of Jas. Martineau: "By an irresistible law of thought _all phenomena present themselves to us as the expression of power_, and refer us to a causal ground whence they issue. This dynamic source we neither see, nor hear, nor feel; it is given in _thought_, supplied by the spontaneous activity of mind as the correlative prefix to the phenomena observed."[251] Unless, then, we are prepared to deny the validity of all our rational intuitions, we can not avoid accepting "this subjective postulate as a valid law for objective nature." If the intuitions of our reason are pronounced deceptive and mendacious, so also must the intuitions of the senses be pronounced illusory and false. Our whole intellectual constitution is built up on false and erroneous principles, and all knowledge of whatever kind must perish by "the contagion of uncertainty." [Footnote 251: "Essays," p. 47.]