A type of reasoning corresponding to the fact that frequently, conclusions that are drawn from a set of premises $\Gamma$ are withdrawn upon the introduction of new formulae (i.e., new information) to $\Gamma$. For example, although one might have grounds to affirm the inference:

• • If Ingrid goes to the zoo, then she will see the lions.

one might reject the inference:

• • If Ingrid goes to the zoo and the great cats exhibit is closed for renovation, then she will see the lions.

Hence, formal models of defeasible reasoning frequently take logical consequence to be non-monotonic, so that it is possible that $\Gamma ⊢\phi$ holds although $\Gamma ,\psi ⊢\phi$ fails, i.e., $\Gamma ,\psi ⊬\phi$.