Full Idea
A tyrannical law, since it is not in accord with reason, is not unconditionally a law, but is rather a perversion of law.
Gist of Idea
Tyrannical laws are irrational, and so not really laws
Source
Thomas Aquinas (Summa Theologicae [1265], Ia IIae.Q92.1, ad 4), quoted by Kretzmann/Stump - Aquinas, Thomas 13
Book Reference
'Shorter Routledge Encyclopaedia of Philosophy', ed/tr. Craig,Edward [Routledge 2005], p.44
A Reaction
Only a belief in natural law can give a basis for such a claim. Positivists will say a tyrannical law is unconditionally a law like any other, but a bad one.