This is the first Economic Policy which boosts something other than Gold or Production. Natural Philosophy is naturally more efficient the more Campuses you have, but it also becomes progressively more efficient the larger their adjacency bonuses are! 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 +3 bonus 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 absolute rather than proportionate, Natural Philosophy will only be worth it if you have more than one Campus and/or 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 into consideration whether or not you really need the Science boost. If you are already managing a fast technical progress, then it will probably be more worthwhile to go with other Economic Policies. On the other hand, if you are playing as the Greeks with their civ ability Plato's Republic (additional Wildcard Policy Card slot in any government), then this card is all but a default peacetime pick. This is particularly the case with Classical Republic, or later alternatively Merchant Republic, as your government.
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.