The substance that cannot be broken down further by chemical means is an element.

Here's why:

* Elements are the simplest pure substances. They are made up of only one type of atom.

* Atoms are the smallest unit of an element that retains the chemical properties of that element.

* Chemical reactions involve the rearranging of atoms, not the breaking down of atoms themselves.

Therefore, since elements are already in their simplest form, they cannot be broken down further by chemical means.