“Rational mechanics must be the science of the motions which result from any forces, and of the forces which are required for any motions, accurately propounded and demonstrated. For many things induce me to suspect, that all natural phenomena may depend upon some forces by which the particles of bodies are either drawn towards each other, and cohere, or repel and recede from each other: and these forces being hitherto unknown, philosophers have pursued their researches in vain. And I hope that the principles expounded in this work will afford some light, either to this mode of philosophizing, or to some mode which is more true.”

Preface
Philosophiae Naturalis Principia Mathematica (1687)

Source: A Bachelor's Establishment (1842), Ch. I.
Context: A grocer is drawn to his business by an attracting force quite equal to the repelling force which drives artists away from it. We do not sufficiently study the social potentialities which make up the various vocations of life. It would be interesting to know what determines one man to be a stationer rather than a baker; since, in our day, sons are not compelled to follow the calling of their fathers, as they were among the Egyptians.

Query 21
Opticks (1704)

On the genesis of two of his historical and autobiographical works.
The Education of Henry Adams (1907)
Context: Any schoolboy could see that man as a force must be measured by motion, from a fixed point. Psychology helped here by suggesting a unit — the point of history when man held the highest idea of himself as a unit in a unified universe. Eight or ten years of study had led Adams to think he might use the century 1150-1250, expressed in Amiens Cathedral and the Works of Thomas Aquinas, as the unit from which he might measure motion down to his own time, without assuming anything as true or untrue, except relation. The movement might be studied at once in philosophy and mechanics. Setting himself to the task, he began a volume which he mentally knew as "Mont-Saint-Michel and Chartres: a Study of Thirteenth-Century Unity." From that point he proposed to fix a position for himself, which he could label: "The Education of Henry Adams: a Study of Twentieth-Century Multiplicity." With the help of these two points of relation, he hoped to project his lines forward and backward indefinitely, subject to correction from any one who should know better. Thereupon, he sailed for home.

[Polymorphisms of the serum proteins and the development of iso-preciptins in transfused patients, Bulletin of the New York Academy of Medicine, 40, 5, 1964, 377–386, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1750599/?page=2] (quote from 378)

"Outlines of Experiments and Inquiries Respecting Sound and Light" (1800)

Definitions - Scholium
Philosophiae Naturalis Principia Mathematica (1687)

Source: Guide for the Perplexed (c. 1190), Part I, p. 296 (1881) Tr. Friedlander

Author, Third Day. Change of Position<!--p.153 [190]-->
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)
Context: My purpose is to set forth a very new science dealing with a very ancient subject. There is, in nature, perhaps nothing older than motion, concerning which the books written by philosophers are neither few nor small; nevertheless I have discovered by experiment some properties of it which are worth knowing and which have not hitherto been either observed or demonstrated. Some superficial observations have been made, as, for instance, that the free motion [naturalem motum] of a heavy falling body is continuously accelerated; but to just what extent this acceleration occurs has not yet been announced; for so far as I know, no one has yet pointed out that the distances traversed, during equal intervals of time, by a body falling from rest, stand to one another in the same ratio as the odd numbers beginning with unity.