"Let me try to explain to you, what to my taste is characteristic for all intelligent thinking. It is, that one is willing to study in depth an aspect of one's subject matter in isolation for the sake of its own consistency, all the time knowing that one is occupying oneself only with one of the aspects. We know that a program must be correct and we can study it from that viewpoint only; we also know that it should be efficient and we can study its efficiency on another day, so to speak. In another mood we may ask ourselves whether, and if so: why, the program is desirable. But nothing is gained —on the contrary!— by tackling these various aspects simultaneously. It is what I sometimes have called "the separation of concerns", which, even if not perfectly possible, is yet the only available technique for effective ordering of one's thoughts, that I know of. This is what I mean by "focussing one's attention upon some aspect": it does not mean ignoring the other aspects, it is just doing justice to the fact that from this aspect's point of view, the other is irrelevant. It is being one- and multiple-track minded simultaneously.

The task of "making a thing satisfying our needs" as a single responsibility is split into two parts "stating the properties of a thing, by virtue of which it would satisfy our needs" and "making a thing guaranteed to have the stated properties". Business data processing systems are sufficiently complicated to require such a separation of concerns and the suggestion that in that part of the computing world "scientific thought is a non-applicable luxury" puts the cart before the horse: the mess they are in has been caused by too much unscientific thought.

Some time ago I visited the computing center of a large research laboratory where they were expecting new computing equipment of such a radically different architecture, that my colleagues had concluded that a new programming language was needed for it if the potential concurrency were to be exploited to any appreciable degree. But they got their language design never started because they felt that their product should be so much like FORTRAN that the casual user would hardly notice the difference "for otherwise our users won't accept it". They circumvented the problem of explaining to their user community how the new equipment could be used at best advantage by failing to discover what they should explain. It was a rather depressing visit.... The proper technique is clearly to postpone the concerns for general acceptance until you have reached a result of such a quality that it deserves acceptance. It is the significance of your message that should justify the care that you give to its presentation, it may be its "unusualness" that makes extra care necessary.