Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class V. Words Releasing to the Voluntary Powers
Division (I) Individual Volition
Section I. Volition in General
1. Acts of Volition

612. Impulse.

   NOUN:IMPULSE, sudden thought; impromptu, improvisation; inspiration, flash, spurt.
  IMPROVISOR, extemporizer, improvvisatore or improvisatore [It.]; creature of impulse.
   VERB:flash on the mind.
  IMPROVISE, extemporize; say what comes uppermost, say what comes first into one’s head; act on the spur of the moment, rise to the occasion; spurt.
   ADJECTIVE:EXTEMPORANEOUS, impulsive, indeliberate [rare]; snap; improvised, improvisate, improviso, improvisatory; unpremediated; unmeditated, improvisatorial, improvisatory, improvisé [F.]; unprompted, unguided; natural, unguarded; spontaneous (voluntary) [See Will]; instinctive [See Necessity].
   ADVERB:EXTEMPORE, extemporaneously; offhand, impromptu, a l’improviste [F.]; on the spur of the -moment, – occasion.
   QUOTATION:To its own impulse every creature stirs.—Arnold