Leonardo da Vinci...on experiments

I shall begin by making some experiments before I proceed any further. For it is my intention first to consult experience and then show by reasoning why that experience was bound to turn out as it did. This, in fact, is the true rule by which the student of natural effects must proceed. Although nature starts from reason and ends with experience, it is necessary for us to proceed the other way around, that is - as I said above - begin with experience and with its help seek the reason. Experience never errs. What alone may err is our judgement, which predicts effects that cannot be produced in our experiments. Given a cause, what follows will of necessity be its true effect, unless some external obstacle intervenes. When that happens, the effect that would have resulted from the cause will reflect the nature of the obstacle in the same proportion as the obstacle is more of less powerful than the cause.

- Notebooks, circa 1500