“Symbols are to the mind what tools are to the hand--an extended application of its powers.”

1940s, The Question – What is your Hope' (c. 1940s)

Source: The Republic of Thieves (2013), Chapter 4 “Across the Amathel” section 3 (p. 190)

Source: The Lost Symbol

30 December 1850
Journal Intime (1882), Journal entries
Context: Everything which is, is thought, but not conscious and individual thought. The human intelligence is but the consciousness of being. It is what I have formulated before: Everything is a symbol of a symbol, and a symbol of what? of mind.

[Life and Scientific Work of Peter Guthrie Tait: supplementing the two volumes of Scientific papers published in 1898 and 1900, Cambridge University Press, 1911, http://www.archive.org/details/lifescientificwo00knotrich, 1]

Source: Science and the Unseen World (1929), Ch. V, p.53

Mind
Source: I am That, P.121.

Federalist No. 48 (1 February 1788) http://en.wikisource.org/wiki/The_Federalist_Papers/No._48
1780s, Federalist Papers (1787–1788)

"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)
Context: In treating of the practical application of scientific principles, an algebraical formula should only be employed when its shortness and simplicity are such as to render it a clearer expression of a proposition or rule than common language would be, and when there is no difficulty in keeping the thing represented by each symbol constantly before the mind.<!--p. 177