In his paper about LSP, uncle Bob mentioned :
Now the rule for the preconditions and postconditions for derivatives, as stated by Meyer, is:
...when redefining a routine [in a derivative], you may only replace its
precondition by a weaker one, and its postcondition by a stronger one.
How could I tell if the preconditions/postconditions of a subtype instance object's method are respectively weaker/stronger than those of the supertype's method?
To formulate it without rigorous definitions:
If your parent class requires something, the child must provide the same functionality - at least. If your routine promises to handle all inputs that are greater than zero, your derived routine must also accept all those, or more inputs. That means that the precondition can only be weaker.
Similarly, the postcondition must be stronger. That means that you are not allowed to return a negative number in your derived routine, if the original routine promised that it will always return a positive number.
If you were to require more than what the parent requires (i.e. if you had a stronger prerequisite), then you could not be sure that you could always call that routine. Let's say that B and C are subclasses of A. Sometimes, you might have an Object of type A, which could be also actually a B or a C. If C had stronger prerequisites than A, you could run into issues when calling the routine on that Object.
I'm sorry if I didn't use the usual terminology, I can't really recall that so I just tried to stick with what makes sense to me. (It's been two years since I last attended a lecture by Bertrand Meyer)