Newton's Principia is written (in Latin) in a lofty, austere way, as if to allow the reader no opportunity to disagree. The Opticks published (in English) 17 years later is more human. It ends with several Queries, in which Newton speculates, without claiming certitude.

I have ventured to borrow Newton's word as the title of this chapter. It is meant to be a warning to the reader that I am now venturing off the fairly well-beaten track followed in the preceding 14 chapters. The speculations that follow are not mine, of course. I have chosen ideas that seem to me to have attracted the attention of the greatest numbers of physicists. I don't suppose any of them is exactly right as it stands. Some may be completely wrong. But I hope that some of them have some truth in them. Only time, and experiment, will tell.

Hidden Dimensions: Charge as Geometry

I begin with a speculation that is almost certainly wrong. My excuses are that it is very pretty and that string theory (Section 15.3) makes use of some of the same ideas.

Around 1915, there seemed to be two beautiful theories: Maxwell's electromagnetism and Einstein's gravity. An obvious dream would be to try to unify them and, in the process, perhaps to find a geometrical basis for electromagnetism similar to Einstein's geometrical theory of gravity. With hindsight, we now know this vision to have been a mirage, because electromagnetism is only a part of the electroweak forces, but even so let us look at one intriguing idea that came up in those days.