There's an old, unglamorous truth about the brain that we keep forgetting because it's inconvenient: it behaves like a muscle. Use it under resistance, and it strengthens. Stop asking it to work, and it weakens — not dramatically, not overnight, but steadily, the way an unused limb loses tone. We understand this instinctively about the body. We seem to keep forgetting it about the mind, especially now that so much of our daily thinking has an off-ramp available at all times.