--From "The Third Anniversary Discourse, on the Hindus," delivered to the Asiatic Society, 2 February 1786. Cited from Sir William Jones : Selected poetical and prose works, ed. by Michael J. Franklin, Cardiff : University of Wales Press, 1995, pp. 355-370.
The Sanscrit language, whatever be its antiquity, is of a wonderful structure; more perfect than the Greek, more copious than the Latin, and more exquisitely refined than either, yet bearing to both of them a stronger affinity, both in the roots of verbs and the forms of grammar, than could possibly have been produced by accident; so strong indeed, that no philologer could examine them all three, without believing them to have spring from some common source, which, perhaps, no longer exists: there is a similar reason, though not quite so forcible, for supposing that both the Gothick and the Celtick, though blended with a very different idiom, had the same origin with the Sanscrit, and the old Persian might be added to this family, if this were the place for discussing any question concerning the antiquities of Persia.
Extra info on Sir William Jones, a fascinating historical figure
"Sir William Jones was one of the greatest polymaths in history. At the time of his early death, in 1794, he knew 13 languages thoroughly and another 28 moderately well. But languages were for him only a means of reaching a deeper understanding, in contrasting cultures, of law, history, literature, music, botany, and other disciplines. Elected at the age of 26 to Johnson's Literary Club and knighted at 37, Jones was a close friend to many leading English luminaries of his time. He was called "Oriental Jones" by some, and his study of middle-eastern cultures, his championship of American independence, and finally his appointment as high court judge in Calcutta, made him a truly universal figure. On the bicentenary of his death, several scholars met at University College, Oxford--his old college--to commemorate his outstanding career and achievements. They found representative themes in Jones's life and work, aiming to strike a balance therein, and to remember, especially, the view taken of Jones by his informed contemporaries. This collection of fascinating papers is a result of that meeting."
--from Sir William Jones, 1746-1794: A commemoration, ed. by Alexander Murray, with an introduction by Richard Gombrich. Oxford and New York: Published on behalf of University College, Oxford, by Oxford University Press, 1998.