Statements: (1) Glass is brittle. (2) This substance is not brittle. Inference: This substance is not glass. The inference is

The inference "This substance is not glass" is definitely true.

"Glass is brittle" means every instance of glass has the property of brittleness.

The substance in hand is stated to lack brittleness.

Whatever lacks a property that all glass has cannot be glass.

Denying the consequent thus denies the antecedent by a valid form.

This is the valid pattern of modus tollens.