All these systems involve the negation of two fundamental scientific principles. The first is that every change must be explained by reference to an antecedent change to which it bears a strict quantitative relation. The second is that no particular change can be referred to another change as its necessary antecedent unless it can be shown by experience that a precisely similar {76} couple of changes are, in fact, always so connected. Let me illustrate these principles by an example. I leave a kettle full of cold water on the fire, and on returning after a sufficient interval of time I find the water boiling. Had I stayed by the fire and watched the process, my kettle would--a popular proverb to the contrary notwithstanding--have certainly boiled as soon, but also no sooner for being helped by my consciousness. The essential thing is that energy of combustion in the fire should be turned into energy of boiling in the water. Now, what is Berkeley's interpretation of the facts? Fire, kettle, water, and ebullition are what in his writings are called "ideas"--_i.e._, phenomena occasionally in my mind, but always in God's mind. And according to this view the necessary antecedent to the boiling of the water is not the fire's burning, but God's consciousness of its burning, his perception being the essence of the operation. But it is proved by experience that neither my perception nor anyone else's ever made a single drop of water boil. In other words, perception is not in this instance a _vera causa_. Why, then, should the perception of any other mind, however exalted, have that effect? Nor is this all. How does Berkeley know that God exists? Because, he says, to exist is to be perceived, and therefore for the universe to exist implies a universal Percipient. But he got the idea of God from other men, who certainly did not come by it as a generalisation from their perceptions; they got it by generalising from their voluntary actions, which do produce the changes that perception cannot produce. It will be said that volitions and the feelings that prompt them exist only in consciousness. In whose consciousness? In that of a spirit. And what is spirit apart from {77} sensation, thought, feeling, and volition? Simply one of those abstract ideas whose existence Berkeley himself denied. HUME.