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One finds in this subject a kind of demonstration which does not carry with it so high a degree of certainty as that employed in geometry; and which differs distinctly from the method employed by geometers in that they prove their propositions by well-established and incontrovertible principles, while here principles are tested by inferences which are derivable from them. The nature of the subject permits of no other treatment. It is possible, however, in this way to establish a probability which is little short of certainty. This is the case when the consequences of the assumed principles are in perfect accord with the observed phenomena, and especially when these verifications are numerous; but above all when one employs the hypothesis to predict new phenomena and finds his expectations realized.
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Christiaan Huygens (1690) Treatise on Light - preface, Translated by Michael R. Matthews, Scientific Background to Modern Philosophy. 1989. p.126

 

Christiaan Huygens

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We know truth, not only by the reason, but also by the heart, and it is in this last way that we know first principles; and reason, which has no part in it, tries in vain to impugn them. The sceptics, who have only this for their object, labour to no purpose. We know that we do not dream, and however impossible it is for us to prove it by reason, this inability demonstrates only the weakness of our reason, but not, as they affirm, the uncertainty of all our knowledge. For the knowledge of first principles, as space, time, motion, number, is as sure as any of those which we get from reasoning. And reason must trust these intuitions of the heart, and must base them on every argument. (We have intuitive knowledge of the tri-dimensional nature of space, and of the infinity of number, and reason then shows that there are no two square numbers one of which is double of the other. Principles are intuited, propositions are inferred, all with certainty, though in different ways.) And it is as useless and absurd for reason to demand from the heart proofs of her first principles, before admitting them, as it would be for the heart to demand from reason an intuition of all demonstrated propositions before accepting them.
This inability ought, then, to serve only to humble reason, which would judge all, but not to impugn our certainty, as if only reason were capable of instructing us. Would to God, on the contrary, that we had never need of it, and that we knew everything by instinct and intuition! But nature has refused us this boon. On the contrary, she has given us but very little knowledge of this kind; and all the rest can be acquired only by reasoning.
Therefore, those to whom God has imparted religion by intuition are very fortunate, and justly convinced. But to those who do not have it, we can give it only by reasoning, waiting for God to give them spiritual insight, without which faith is only human, and useless for salvation.

 

Blaise Pascal

 

I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto attempted the search of nature in vain; but I hope the principles here laid down will afford some light either to that or some truer method of philosophy.

 

Isaac Newton

 

Our design, not respecting arts, but philosophy, and our subject, not manual, but natural powers, we consider chiefly those things which relate to gravity, levity, elastic force, the resistance of fluids, and the like forces, whether attractive or impulsive; and therefore we offer this work as mathematical principles of philosophy; for all the difficulty of philosophy seems to consist in this — from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena...

 

Isaac Newton

 

From principles is derived probability, but truth or certainty is obtained only from facts.

 

Tom Stoppard

 

Hypothesis Of Molecular Vortices. In thermodynamics as well as in other branches of molecular physics, the laws of phenomena have to a certain extent been anticipated, and their investigation facilitated, by the aid of hypotheses as to occult molecular structures and motions with which such phenomena are assumed to be connected. The hypothesis which has answered that purpose in the case of thermodynamics, is called that of "molecular vortices," or otherwise, the "centrifugal theory of elasticity. (On this subject, see the Edinburgh Philosophical Journal, 1849; Edinburgh Transactions, vol. xx.; and Philosophical Magazine, passim, especially for December, 1851, and November and December, 1855.)

 

William John Macquorn Rankine