I call a cause, in the narrowest sense of the word, that state of matter, which, while it introduces another state with necessity, yet suffers just as great a change itself as that which it causes; which is expressed in the rule, "action and reaction are equal."
Nik kausa deitzen diot hitzaren zentzurik hertsienean materiaren halako egoerari, zeinak beste bat beharrezkotasunarekin sortzen duelarik, berak kausatu duen bezain aldaketa handia jasaten baitu, zeina arauaren bitartez adierazten baita.