A selection of excerpts from Steven Weinberg’s “Dreams of a Final Theory”.

1. Deduction $\neq$ Explanation. Saying that one truth (e.g., quantum mechanics) explains another (e.g., laws of chemistry) does not always mean we can sit down and logically deduce every detail from first principles. Often, the systems are too complex for such deductions to be feasible. Scientific explanation often refers not to what we can actually work out, but to what the structure of nature compels.

   When we say that one truth explains another, as for instance that the physical principles (the rules of quantum mechanics) governing electrons in electric fields explain the laws of chemistry, we do not necessarily mean that we can actually deduce the truths we claim have been explained. Sometimes we can complete the deduction, as for the chemistry of the very simple hydrogen molecule. But sometimes the problem is just too complicated for us. In speaking in this way of scientific explanations, we have in mind not what scientists actually deduce but instead a necessity built into nature itself. For instance, even before physicists and astronomers learned in the nineteenth century how to take account of the mutual attraction of the planets in accurate calculations of their motions, they could be reasonably sure that the planets move the way they do because they are governed by Newton’s laws of motion and gravitation, or whatever more exact laws Newton’s laws approximate. Today, even though we cannot predict everything that chemists may observe, we believe that atoms behave the way they do in chemical reactions because the physical principles that govern the electrons and electric forces inside atoms leave no freedom for the atoms to behave in any other way.

2. At some point, eminent physicists like Albert Michelson, Philipp von Jolly, and Lord Kelvin, believed that there was nothing new to be discovered in physics expect making measurements more and more precise. For instance, Albert Michelson at a talk in University of Chicago in 1894 said,

   While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. An eminent physicist (Lord Kelvin) has remarked that the future truths of Physical Science are to be looked for in the sixth place of decimals.

Additionally,

When the young Max Planck entered the University of Munich in 1875, the professor of physics, Jolly, urged him against studying science. In Jolly’s view there was nothing left to be discovered.