9. A maxim which it may be useful to recollect is this;--that _hypotheses may often be of service to science, when they involve a certain portion of incompleteness, and even of errour_. The object of such inventions is to bind together facts which without them are loose and detached; and if they do this, they may lead the way to a perception of the true rule by which the phenomena are associated together, even if they themselves somewhat misstate the matter. The imagined arrangement enables us to contemplate, as a whole, a collection of special cases which perplex and overload our minds when they are considered in succession; and if our scheme has so much of truth in it as to conjoin what is really connected, we may afterwards duly correct or limit the mechanism of this connexion. If our hypothesis renders a reason for the agreement of cases really similar, we may afterwards find this reason to be {84} false, but we shall be able to translate it into the language of truth. A conspicuous example of such an hypothesis,--one which was of the highest value to science, though very incomplete, and as a representation of nature altogether false,--is seen in the _Doctrine of epicycles_ by which the ancient astronomers explained the motions of the sun, moon, and planets. This doctrine connected the places and velocities of these bodies at particular times in a manner which was, in its general features, agreeable to nature. Yet this doctrine was erroneous in its assertion of the _circular_ nature of all the celestial motions, and in making the heavenly bodies revolve _round the earth_. It was, however, of immense value to the progress of astronomical science; for it enabled men to express and reason upon many important truths which they discovered respecting the motion of the stars, up to the time of Kepler. Indeed we can hardly imagine that astronomy could, in its outset, have made so great a progress under any other form, as it did in consequence of being cultivated in this shape of the incomplete and false _epicyclical hypothesis_.