Because of its nature, the inner layer is totally yours. Which means, you have 100% control over it.
Whatever happens in the outer layer, you can think whatever you want in the inner layer. It is true! However, usually people react to the outer layer, such as environments, conditions, circumstances. When it rains, you feel sad. Why? Because you "think" rain is somehow bad. Probably you have a plan to go picnic or something. When it rains, you feel good. Because you think rain is good. You are growing vegetables in your garden.
It is a simple fact but being overlooked most of the time by most people. Your inner layer is YOURS!