What is an institution?

When I was an undergraduate in Oxford, we were taught economics almost as though it were a natural science. The subject matter of economics might be different from physics, but only in the way that the subject matter of chemistry or biology is different from physics. The actual results were presented to us as if they were scientific theories. So, when we learned that savings equals investment, it was taught in the same tone of voice as one teaches that force equals mass times acceleration. And we learned that rational entrepreneurs sell where marginal cost equals marginal revenue in the way that we once learned that bodies attract in a way that is directly proportional to the product of their mass and inversely proportional to the square of the distance between them. At no point was it ever suggested that the reality described by economic theory was dependent on human beliefs and other attitudes in a way that was totally unlike the reality described by physics or chemistry.