Now, let any philosopher make a trial, and endeavour to explain that act of the mind which we call _belief_, and give an account of the principles from which it is derived, independent of the influence of custom on the imagination, and let his hypothesis be equally applicable to beasts as to the human species; and, after he has done this, I promise to embrace his opinion. But, at the same time I demand as an equitable condition, that if my system be the only one, which can answer to all these terms, it may be received as entirely satisfactory and convincing. And that 'tis the only one, is evident almost without any reasoning. Beasts certainly never perceive any real connexion among objects. 'Tis therefore by experience they infer one from another. They can never by any arguments form a general conclusion, that those objects of which they have had no experience, resemble those of which they have. 'Tis therefore by means of custom alone that experience operates upon them. All this was sufficiently evident with respect to man. But with respect to beasts there cannot be the least suspicion of mistake; which must be owned to be a strong confirmation, or rather an invincible proof of my system. Nothing shows more the force of habit in reconciling us to any phenomenon, than this, that men are not astonished at the operations of their own reason, at the same time that they admire the _instinct_ of animals, and find a difficulty in explaining it, merely because it cannot be reduced to the very same principles. To consider the matter aright, reason is nothing but a wonderful and unintelligible instinct in our souls, which carries us along a certain train of ideas, and endows them with particular qualities, according to their particular situations and relations. This instinct, 'tis true, arises from past observation and experience; but can any one give the ultimate reason, why past experience and observation produces such an effect, any more than why nature alone should produce it? Nature may certainly produce whatever can arise from habit: nay, habit is nothing but one of the principles of nature, and derives all its force from that origin. PART IV. OF THE SCEPTICAL AND OTHER SYSTEMS OF PHILOSOPHY. SECTION I. OF SCEPTICISM WITH REGARD TO REASON.