Thermometers are useful tools. They assist in determining whether a loved one is sick by detecting (or confirming) the presence of a fever. These are not to be confused with cooking thermometers which assist in making sure food is cooked thoroughly to keep your family healthy in the first place. Either way, they are efficient at what they are designed to do which is purely a measurement of what is.
Contrast this with a thermostat which has an additional feature. After assessing what is (the current temperature), thermostats have the capacity to change the environment and make the room warmer or cooler (what can/will be).
As Christians, we are called to assess what is and to then improve the culture around us. This is a radical and counter-cultural concept. St. Francis of Assisi grasped how this would work in the prayer associated with him.Prayer of Saint Francis of Assisi
Lord, make me an instrument of thy peace! That where there is hatred, I may bring love. That where there is wrong, I may bring the spirit of forgiveness. That where there is discord, I may bring harmony. That where there is error, I may bring truth. That where there is doubt, I may bring faith. That where there is despair, I may bring hope. That where there are shadows, I may bring light. That where there is sadness, I may bring joy. Lord, grant that I may seek rather to comfort, than to be comforted. To understand, than to be understood. To love, than to be loved. For it is by self-forgetting that one finds. It is by forgiving that one is forgiven. It is by dying that one awakens to Eternal Life. —Saint Francis of Assisi—
When Jesus called us to be salt and light in this world, he was not calling us to be a thermometer noting how unhealthy we are, but to act as a thermostat and bring wholeness to a sick and dying world.