HYPOTHESIS, in Physics, &c. denotes a kind of system laid down from our own imagination, whereby to account for some phenomenon or appearance of nature. Thus we have hypotheses to account for the tides, for gravity, for magnetism, for the deluge, &c.
The real and scientific causes of natural things generally lie very deep: observation and experiment, the proper means of arriving at them, are in most cases extremely slow, and the human mind is very impatient: hence we are frequently driven to feign or invent something that may seem like the cause, and which is calculated to answer the several phenomena, so that it may possibly be the true cause.
Philosophers are divided as to the use of such fictions or hypotheses, which are much less current now than they were formerly. The latest and best writers are for excluding hypotheses, and standing wholly on observation and experiment. Whatever is not deduced from phenomena, says Sir Isaac Newton, is an hypothesis; and hypotheses, whether metaphysical, or physical, or mechanical, or of occult qualities, have no place in experimental philosophy.
The Cartesians take upon them to suppose what affections in the primary particles of matter they please; just what figures, what magnitudes, what motions, and what situations, they find for their purpose. They also feign certain unseen, unknown fluids, and endue them with the most arbitrary properties; give them a subtlety which enables them to pervade the pores of all bodies, and make them agitated with the most unaccountable motions. But is not this to set aside the real constitution of things, and to substitute dreams in their place? Truth is scarce attainable even by the surest observations; and will fanciful conjectures ever come at it? They who found their speculations on hypotheses, even though they argue from them regularly, according to the strictest laws of mechanics, may be said to compose an elegant and artful fable; but it is still only a fable.