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object to which the exercised mind of a true Newtonian disciple is more familiarized than this limit; and it serves as a boundary by which he shapes, and bounds, and regulates all the enterprises of his philosophy. All the space which lies within this limit, he cultivates to the uttermost; and it is by such successive
; labours, that every year which rolls over the world, is witnessing some new contribution to experimental science, and adding to the solidity and aggrandizement of this wonderful fabric. But, if true to their own principle, then, in reference to the forbidden ground which lies without this limit, those very men, who, on the field of warranted exertion, evinced all the hardihood and vigour of a full grown
understand ing, show, on every subject where the light of evidence is withheld from them, all the modesty of children. They give you positive opinion
. only when they have indisputable proof-but, when they have no such proof, then they have no such opinion. The single principle of their respect to truth, secures their homage for every one position, where the evidence of truth is present; and at the same time, begets an entire
diffidence about every one position, from which this evidence is disjoined. And thus you may understand, how the first man in the accomplishments of philosophy, which the world ever saw, sat at the book of nature in the humble attitude of its interpreter and its pupil-how all the docility of conscious ignorance threw a sweet and softening lustre around the radiance even of his most splendid discoveries-and, while the flippancy of a few superficial acquirements is enough to place a philosopher of the day on the pedestal of his fancied elevation, and to vest him with an assumed lordship over the whole domain of natural and revealed knowledge; I cannot forbear to do honour to the unpretending greatness of Newton, than whom I know not if ever there lighted on the face of our world, one in the character of whose admirable genius so much force and so much humility were more attractively blended.
I now purpose to carry you forward, by a few simple illustrations, to the argument of this day. All the sublime truths of the modern astronomy lie within the field of actual observation, and have the firm evidence to rest upon of all that information which is conveyed to us by the avenue of the senses. Sir Isaac Newton never went beyond this field, without a reverential impression upon his mind, of the precariousness
, of the ground on which he was standing. On this ground, he never ventured a positive affirmation--but, resigning the lofty tone of demonstration, and putting on the modesty of conscious ignorance, he brought forward all he had to say in the humble form of a doubt, or a conjecture, or a question. But, what he had not confidence to do, other philosophers have done after him—and they have winged their audacious way into forbidden regions--and they have crossed that circle by which the field of observation is enclosed—and there have they debated and dogmatized with all the pride of a most intolerant assurance,
Now, though the case be imaginary, let us conceive for the sake of illustration, that one of these philosophers made so extravagant a departure from the sobriety of experimental science as to pass on from the astronomy of the different planets, and to attempt the natural history of their animal and vegetable kingdoms. He might get hold of some vague and general analogies, to throw an air of plausibility around his speculation. He might pass from the botany of the different regions of the globe that we inhabit; and make his loose and confident applications to each of the other planets, according to its distance from the sun, and the inclination of its axis to the plane of its annual revolution ; and out of some such slender materials, he
may work up an amusing philosophical romance, full of ingenuity, and having, withal, the colour of truth and of consistency spread over it.
I can conceive how a superficial public might be delighted by the eloquence of such a composition, and even to be impressed by its arguments; but were I asked, which is the man of all the ages and countries in the world, who would have the least respect for this treatise upon the plants which grow on the surface of Jupiter, I should be at no loss to answer the question. I should say, that it would be he who had computed the motions of Jupiter
that it would be he who had measured the bulk and the density of Jupiter that it would be he who had estimated the periods of Jupiter-that it would be he whose observant eye and patiently calculating mind, had traced the satellites of Jupiter through all the rounds of their mazy circulation, and unravelled the intricacy of all their movements. He would see at once that the subject lay at a hopeless distance beyond the field of legitimate observation. It would be quite enough for him, that it was beyond the range of his telescope. On this ground, and on this ground only, would he reject it as one of the puniest imbecilities of childhood. As to any character of truth or of importance, it would have no more effect on such a mind as that of Newton, than any illusion of poetry; and, from the eminence of his intellectual throne, would he cast a penetrating glance at the whole speculation, and bid its gaudy insignificance away from him. .
But let us pass onward to another case, which though as imaginary as the former, may still serve the purpose of illustration.