Newton didn't "discover" the second law of motion in the sense of finding it like a lost treasure. Instead, he formulated it based on observations, experiments, and the work of others before him.

Here's how his process likely unfolded:

1. Building on Prior Knowledge: Newton was well-versed in the work of Galileo Galilei, who had already established the relationship between force and acceleration through his experiments with inclined planes and falling objects.

2. Observations and Experiments: Newton conducted his own experiments, like dropping objects from different heights and observing their motion. He also studied the motion of planets and the moon.

3. Mathematical Reasoning: Newton used his mathematical skills to analyze the data he collected and formulate a precise mathematical relationship between force, mass, and acceleration. He used calculus, which he also invented, to describe motion in a more precise way than his predecessors.

4. Developing a Universal Law: Newton was able to go beyond Galileo's work by realizing that the same laws governing motion on Earth also applied to celestial bodies. This was a groundbreaking realization that unified terrestrial and celestial mechanics.

It's important to remember that scientific discoveries are often a process of building on the work of others, making observations, conducting experiments, and formulating laws based on those observations. Newton's second law of motion was the culmination of this process, bringing together the insights of many other scientists before him.