Validating ONLY with tests is basically flying the plane on instrumentation, versus being able to look out the windshield. Flying visually and by muscle-memory is both more efficient and safer, in conjunction with instrumentation. You’re much less likely to hit a mountain by mistake.
When you’ve been coding for more than twenty years, it can be difficult to recapture beginner’s mind, and explain how to think like a programmer to someone who is new to it. I remember an incident in college, when I had been coding for a comparatively short time, that crystalized in my mind the thought process behind writing code—what you might call the programmer philosophy.