He thought of a chemical reaction as always the same under the same conditions, as an action between a fixed and measurable quantity of one substance, having definite and definable properties, with fixed and measurable quantities of other substances, the properties of each of which were definite and definable.
Lavoisier also recognised that certain definite substances could be divided into things simpler than themselves, but that other substances refused to undergo simplification by division into two or more unlike portions.

He spoke of the object of chemistry as follows:--[8] "In submitting to experiments the different substances found in nature, chemistry seeks to decompose these substances, and to get them into such conditions that their various components may be examined separately.

Chemistry advances to its end by dividing, sub-dividing, and again sub-dividing, and we do not know what will be the limits of such operations.

We cannot be certain that what we regard as simple to-day is indeed simple; all we can say is, that such a substance is the actual term whereat chemical analysis has arrived, and that with our present knowledge we cannot sub-divide it." [8] I have given a free rendering of Lavoisier's words.
In these words Lavoisier defines the chemical conception of _elements_; since his time an element is "the actual term whereat chemical analysis has arrived," it is that which "with our present knowledge we cannot sub-divide"; and, as a working hypothesis, the notion of _element_ has no wider meaning than this.

I have already quoted Boyle's statement that by _elements_ he meant "certain primitive and simple bodies ...