the brain takes its shape from what the mind rests upon.

As they say in neuroscience: Neurons that fire together wire together. Mental states become neural traits.

positive experiences use standard-issue memory systems, in which new information must be held in short-term buffers long enough for it to transfer to long-term storage. “Long enough” depends on the experience and the person, but loosely speaking it’s at least a few seconds, and the longer the better.

Because they lived in small bands, it was uncommon for them to meet people they didn’t know, and often dangerous when they did. While some bands interacted peacefully with each other, on average about one in eight men died in conflicts between bands, compared to one in a hundred men who died due to warfare in the twentieth century.

Our ancestors could make two kinds of mistakes: (1) thinking there was a tiger in the bushes when there wasn’t one, and (2) thinking there was no tiger in the bushes when there actually was one. The cost of the first mistake was needless anxiety, while the cost of the second one was death.

mindfulness meditators have increased gray matter—which means a thicker cortex—in three key regions: prefrontal areas behind the forehead that control attention; the insula, which we use for tuning into ourselves and others; and the hippocampus.

Over time, negative experiences make the amygdala even more sensitive to the negative. This snowballing effect occurs because the cortisol that the amygdala signals the hypothalamus to call for enters the bloodstream and flows into your brain, where it stimulates and strengthens the amygdala.

the cortisol in your brain overstimulates, weakens, and eventually kills cells in your hippocampus, gradually shrinking it.

Unlike fleeting mental states, inner strengths are stable traits, an enduring source of well-being, wise and effective action, and contributions to others.