missed, cross-references have been inserted. The selections from
the Optics (i.e., the Questions from Book III) allow the reader to
follow Newton's own arrangement and development of those
thoughts which concerned him most deeply. In these Questions, it
will be observed, almost all of the fundamental ideas which occu-
pied Newton--and which make up the body of the selections in this
book--appear again, but within the ordered sequence of the spec-
ulative reflections to which he gave such brilliant articulation.

I must assume full responsibility for the particular selections
included, their arrangement, and the historical and explanatory
Notes at the end of the book. Where it has seemed necessary to
add a few words either to the text or as footnotes, the additions
appear in brackets. Newton's punctuation and spelling have been
altered here and there to conform to current usage. The Bibliog-
raphy lists the original sources of the selections, as well as a num-
ber of valuable secondary sources.

I am grateful to the University of California Press for permission
to use Florian Cajori's excellent revision of Motte translation of
the Principia. Cajori's historical appendix is of importance; his
revision of the text, in rendering the now obsolete mathematical
terminology into modern equivalents, makes the California edition
indispensable to the modern reader of Newton's masterpiece. My
thanks go also to Professor E. A. Burtt, who has kindly granted
me permission to quote, in the note to the General Scholium, from
his justly well-known and valuable book The Metaphysical
Foundations of Modern Physical Science. I have learned much
from L. T. More's important and informative biography of New-
ton.
I wish to acknowledge my indebtedness to Dr. Jason L. Saun-
ders for his careful preliminary study and subsequent translation
from the Latin of Newton's letter to Oldenburg on hypotheses. To
Professors Ernest Nagel, Marjorie H. Nicolson, and Herbert W.
Schneider, I am also grateful for valuable suggestions and advice.

H. S. THAYER

COLUMBIA UNIVERSITY

September, 1952

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