To him that has, more shall be given; but from him that has not, the little that he has shall be taken away. —St. Matthew
Some ideas acquire undue influence. The prominence of the body-support idea is well-deserved; no other scheme compares to its ability to help us link things into causelike chains. But there are other, not so honorable ways for ideas to gather influence.
The Investment Principle: Our oldest ideas have unfair advantages over those that come later. The earlier we learn a skill, the more methods we can acquire for using it. Each new idea must then compete against the larger mass of skills the old ideas have accumulated.
This is why it's so much easier to do new things in older ways. Each new idea, however good in principle, seems awkward until we master it. So old ideas keep gaining strength, while new ones can rarely catch up. Furthermore, our oldest and best-developed skills will be the first to spread to other realms of thought where again they'll start out far enough ahead to keep any new ideas from taking root.
In the short run, you will usually do better by using an old idea than by starting out anew. If you can already play the piano well, it is easy to start playing the organ in the same way. The many superficial similarities will make it hard for you to tell which aspects of your old skills are unsuitable, and the easiest course is to keep applying your old technique, trying to patch each flaw until none show. In the long run, you'd probably do better by starting fresh with a new technique — and then borrowing what you can from your older skills. The trouble is that we're almost always immersed in the short run. So the principles both of investment and of exception make us reluctant to tamper with our well-established skills and uniframes lest we endanger all that we have built upon those old foundations. I don't mean to say there's anything wrong, in principle, with using what you are comfortable with and already know. But it is dangerous to support your old ideas merely by accumulating ways to sidestep their deficiencies. That only increases the power of your old ideas to overcome new ones and could lead your style of thought to base itself yet all the more, as time goes by, upon less and less.
Evolution illustrates how processes can become enslaved by the investment principle. Why do so many animals contain their brains inside their heads — as with fish, amphibians, reptiles, birds, and bats? This arrangement was inherited long before our earliest aquatic ancestor first crawled upon the land three hundred million years ago. For many of those animals — woodpeckers, for example — another arrangement might serve at least as well. But once the pattern of centralizing so many functions in the head was established, it carried with it great networks of dependencies involving many aspects of anatomy. Because of this, any mutation that changed any part of that arrangement would disrupt many other parts and lead to dreadful handicaps, at least in the short run of evolution. And because evolution is so inherently short-sighted, it would not help if, over longer spans of time, such changes could lead to advantages. Perhaps the best example of this can be seen in the fact that virtually every detail of every plant and animal on earth is written in terms of a genetic code that has scarcely changed a single bit in a billion years. It does not seem to be a particularly efficient or reliable code, yet so many structures have been based on it that all living things are stuck with it! To change a single detail of that code would cause so many proteins to get tangled up that not a single cell could live.