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

Perfect reusable components are not obtained at the first shot.

C++ is the only current language making COBOL look good

Constants are widely known for the detestable practice of changing their values; we should prepare ourselves against the consequences of such fickleness

Ask not first what the system does; ask what it does it to!

Careful as they may be, developers of Eiffel libraries will always run into cases in which, after releasing a library class, they suddenly experience what in French is called esprit de l’escalier or wit of the staircase: a great thought which unfortunately is an afterthought.

As Mr. Nagle so competently points out, almost no one uses Eiffel; in fact until recently there were only 9 users. But now a 10th person just started, so we are holding a conference, appropriately titled the TENTH Eiffel USER conference, to celebrate.

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

I have always felt sympathy towards the biologists who accept to debate creationists. Now I also understand them better; one can fight opinions, not articles of faith.

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.

I think Smalltalk is inappropriate for serious industrial developments. After all, run time is a little late to find out whether you have a landing gear.

Incorrect documentation is often worse than no documentation.

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.

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.

Like a clever reply that would have stunned all the other dinner guests – if only you had thought of it before walking down the stairs after the party is over