The System of Nature, Vol. 2

As this agent was invisible, as his mode of action was inconceivable, he made him a spirit, a word that really means nothing more than that he is ignorant of his essence, or that he acts like the breath of which he cannot trace the motion. Thus, in speaking of spirituality, he designated an occult quality, which he deemed suitable to a concealed being, whose mode of action was always imperceptible to the senses.