There are several reasons why PSI uses the Physics-Chemistry-Biology sequence.

First, in order to understand modern Biology, students need to understand Chemistry, and to understand Chemistry, students must understand Physics. Therefore, Physics is a prerequisite of Chemistry, and Chemistry is a prerequisite of Biology. This is easy to see in textbooks if you definite prerequisite as what is required to understand the topics in a textbook, but that which is not in the book. An examination of textbooks shows that physics has no prerequisite chemistry or biology, chemistry has no prerequisite biology but some physics, and biology has the prerequisite of chemistry and physics.

Second, mathematics aligns better with Physics at the beginning of the science sequence. Success in Algebra I by the end of 9th grade has been highly correlated with successful graduation from college. Biology does not assist students with learning Algebra I, but an Algebra-based Physics provides students a natural venue for practicing algebra. While students do not utilize all aspects of algebra in such a course, they do practice a number of topics deeply.

Third, this sequence maximizes the number of Advanced Placement (AP) science courses to students and increases subject continuity. While all students take the core Physics-Chemistry-Biology sequence, students can take AP Physics B while in their second year concurrently with Chemistry, AP Chemistry during their third year concurrently with Biology, and AP Biology in their fourth year. Each AP follows the first year course, avoiding gaps between the first year version of the course and the AP.