A thermometer is circumstantially led. Depending on the temperature, the mercury moves up or down. It moves according to the circumstances that it is set in. Everything around a thermometer controls how the thermometer works, whether it moves up or down.
A thermostat influences its surroundings. Unlike the thermometer, the thermostat controls the heat or coldness around it.
Which are you? Do you influence your surroundings? Or, are you influenced by your surroundings?