This is the first Economic Policy which boost something different than Gold or Production. Natural Philosophy is naturally more efficient the more Campuses you have, but not only that - it also becomes progressively more efficient the larger their Adjacency bonuses! For example, a Campus with a bonus of +1 will gain another 1 from this Policy, for a total of 2; while a Campus with a bonus +3 will reach a total of 6! Since in Civilization VI every single point of a yield type counts, and most of the time the gain is not proportionate, but absolute, then Natural Philosophy will only be worth it if you have more than 1 Campus, and/or if your Campuses have good Adjacency bonuses! Otherwise, it won't be worth it to spend an Economic slot just to gain a modest 1 or 2 Science.
Of course, you will also have to take in mind whether you really need the Science boost. If you are already managing a fast technical progress, then it will probably be more worth it to go with other Economic Policies.
Civilopedia Entry Edit
From the days of Aristotle, natural philosophy was the study of the physical world, the first systematic “science.” Every other science – biology, physics, chemistry, astronomy, mathematics, and all – has its roots in natural philosophy, and most of Aristotle’s predecessors such as Parmenides, Heraclitus, and Democritus centered their beliefs on the natural order. Aristotle’s conceptions of the laws that govern the real world, set down in the Corpus Aristotelicum (the bits that survived), prevailed into the Middle Ages. Although the empirical study and understanding of everything would separate into various scientific disciplines, it all began with natural philosophy. And now the boundaries between the disciplines are getting blurry again.