the nearer to truth the more they approach to diftinctness, and that the most confufed representations are the moft falfe. It was not by reafoning about the fallacy of the senses, and profecuting a train of argument beyond the principles of common fenfe, that men difcovered the true fyftem of the world. In the earlier ages, when they imagined the fun to be little bigger than the mountain beyond which he difappeared, it was abfurd to think of the earth revolving round him. But in procefs of time, ingenious men, who applied themselves to the obfervation of the heavenly bodies, not with a view to confute popular errors, for they could not as yet even fufpect the vulgar opinion to be erroneous, but merely to gratify their own laudable curiofity, began to conceive more exalted notions of the mundane fyftem. They foon diftinguished the planets from the fixed stars, by obferving the former to be more variable in their appearances. After a long fucceffion of years, they came at laft to understand the motions of the fun and moon fo well, that, to the utter aftonishment of the vulgar, they began to calculate eclipfes a degree of knowledge they could not attain, without being convinced, that the fun and moon are very large bodies, placed at very great diftances from the earth, the former much larger, and more remote, than the latter. Thus far it is impoffible to show, that any reafoning had been employed : by those ancient aftronomers, either to prove, or to difprove, the evidence of the fenfes. On the contrary, they must all along have taken it for granted, that the fenfes are not fallacious; fuppofing only, (what it is certainly agreeable to common fenfe to fuppofe), that the experience of a diligent obferver is more to be depended on than that of the inattentive multitude. As men grew more and more acquainted with the motions and appearances of the heavenly bodies, they became more and more fenfible, that the fun, earth, and planets, bear fome very peculiar relation to one another: and having learned from the phenomena of eclipfes, and fome other natural appearances, that the fun is bigger than the earth *, they might, without abfurdity, begin to fufpect, that poffibly the fun might be the centre round which the earth and other planets revolve; especially confidering the magnificence of that glorious luminary, and the wonderful and delightful effects produced by the influence of his *Heraclitus maintained, that the fun is but a foot broad; Anaxagoras, that he is much larger than the country of Peloponnefus; and Epicurus, that he is no bigger than he appears to the eye. But the astronomers of antiquity maintained, that he is bigger than the earth; eight times, according to the Egyptians; eighteen times, according to Eratofthenes; three hundred times, according to Cleomedes; one thoufand and fifty times, according to Hipparchus; and fifty-nine thoufand three hundred and nineteen times, according to Poffidonius. VOL. I. Y beams, beams, while at the fame time he feems not to derive any advantage from the earth, or other planets. But if the matter had been carried no further, no reasoning from thefe circumftances could ever have amounted to a proof of the point in question, though it might breed a faint prefumption in its favour. For fill the evidence of sense seemed to contradict it; an evidence that nothing can difprove, but the evidence of fenfe placed in circumstances more favourable to accurate obfervation. The invention of optical glaffes did at laft furnish the means of making experiments with regard to this matter, and of putting man in circumftances more favourable to accurate obfervation; and thus the point was brought to the test of common fenfe. And now, we not only know, that the Copernican theory is true, for every perfon who understands it is convinced of its truth; but we alfo know to what causes the univerfal belief of the contrary doctrine is to be afcribed. We know that men, confidering the remote fituation of our earth, and the imperfection of our fenfes, could not have judged otherwife than they did, till that imperfection was remedied, either by accuracy of obfervation, or by the invention of optical inftruments. We fpeak not of revelation; which has indeed been vouchfafed to man for the regulation of his moral conduct; but which it would be prefumption to expect, or defire, defire, merely for the gratification of curiofity. It it evident, from what has been said, that in natural philofophy, as well as in mathematics, no argumentation is profecuted beyond self-evident principles; that as in the latter all reasoning terminates in intuition, fo in the former all reafoning terminates in the evidence of fenfe. And as, in mathematics, that is accounted an intuitive axiom, which is of itself fo clear and evident, that it cannot be illustrated or enforced by any medium of proof, and which must be believed, and is in fact believed, by all, on its own authority; so, in natural philosophy, that is accounted an ultimate principle, undeniable and unquestionable, which is fupported by the evidence of a well-informed fenfe, placed fo as to perceive its object. In mathematics, that is accounted falfe doctrine which is inconsistent with any felf-evident principle; in natural philofophy, that is rejected which contradicts matter of fact, or, in other words, which is repugnant to the appearances of things as perceived by external fenfe. Regulated by this criterion of truth, mathematics and natural philosophy have become of all sciences the most refpectable in point of certainty. Hence I am encouraged to hope, that if the fame criterion were univerfally adopted in the philofophy of the mind, the fcience of human nature, instead of being, as at prefent, a chaos of uncertainty and contradiction, would acquire a confiderable degree of certainty, perfpicuity, and order. If truth be at all attainable in this fcience, (and if it is not attainable, why fhould we trouble our heads about it?), furely it must be attained by the fame means as in those other fciences. I therefore would propofe, "That in the philofophy of human nature, as well as in phyfics and mathematics, principles be examined according to the standard of common fenfe, and be admitted or rejected as they are found to agree or disagree "with it: more explicitly, "That those "doctrines be rejected which contradict matter of fact, that is, which are repug nant to the appearances of things, as per"ceived by external and internal fense; and "that those principles be accounted ulti mate, undeniable, and unquestionable, "which are warranted by the evidence of a "well-informed sense, placed in circumstances favourable to a diftinct perception of "its object.” But what do you mean by a well-informed fenfe? How fhall I know, that any particular faculty of mine is not defective, depraved, or fallacious? Perhaps it is not easy, at least it would furnish matter for too long a digreffion, to give a full answer to this queftion. Nor is it at prefent neceffary; because it will appear in the fequel, that, however difficult |