Two things that many great putters do innately that seemingly have basically gone unnoticed over the years are observation from a distance of green contours and close inspection of the path of ALL shots hit onto the green surface. A player (particularly if walking the course) from 50-100 yards out can not help but notice the multiple contours that are present on most greens. I think many good putters did this without even knowing it. The human brain catalogued what the modern green books illustrated. The second thing that the curious and/or observant golfer gained valuable information from was watching ALL shots hit to the green roll out. Only illegally rolling a ball on the green before putting could be more valuable. Even seeing shots not hit on the same line as your putt give usable info. Could be 2 components of “feel”.