Sometimes I'm afraid I'm losing my ability to think. The source of that fear, I believe, is working in isolation.
When I taught, I had plenty of stimulation, and I wasn't afraid of what I didn't know. Colleagues listened to what I had to say and encouraged me to explore ideas. They challenged me to examine my intellectual processes, mentored me, asked for my opinions about their ideas, introduced me to concepts I would not have been aware of otherwise. And they expected me to challenge them. There was tedium. But to offset it, there was competition and there was synergism.
When you work with others on a regular basis, both that competition and that synergy are part of your natural working environment. When you work in isolation, if you don't create them, they aren't there.