As useless laws weaken necessary laws, those that can be evaded weaken legislation.

[Comme les lois inutiles affaiblissent les lois nécessaires, celles qu’on peut éluder affaiblissent la législation.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Spirit of Laws [The Spirit of the Laws; De l’esprit des lois], Book 29, ch. 16 (1748) [tr. Cohler/Miller/Stone (1989)]
    (Source)

(Source (French)). Other translations:

As useless laws debilitate such as are necessary, so those that may be easily eluded weaken the legislation.
[tr. Nugent (1750)]

As needless laws weaken necessary laws, laws that can be eluded weaken legislation.
[tr. Stewart (2018)]

Variant: "Useless laws weaken the necessary laws."

The same thought, that "Useless laws weaken necessary laws [Les loix inutiles affaiblissent celles qui sont les necessaires.]" also is recorded in his Pensées Diverses [Assorted Thoughts], # 630 / 1007 "General Maxims of Politics," No. 6 (1720-1755). Variant: "Les loix inutiles affaiblissent les necessaires."

Other translations of that work:

Useless laws debilitate such as are necessary.
[ed. Guterman (1963)]

Useless laws weaken necessary ones.
[tr. Clark (2012)]