Writing a class without its contract would be similar to producing an engineering component (electrical circuit, VLSI (Very Large Scale Integration) chip, bridge, engine...) without a spec. No professional engineer would even consider the idea.

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

The role of a trainer or consultant is to empower the customer, not to make himself indispensable

You can either have software quality or you can have pointer arithmetic, but you cannot have both at the same time.

Eiffel borrows quite openly from several earlier programming languages and I am sure that if we had found a good language construct in C we would have used it as well.

Correctness is clearly the prime quality. If a system does not do what it is supposed to do, then everything else about it matters little.

Incorrect documentation is often worse than no documentation.