Thought breathes through action.
Thinking and doing, doing and thinking—these are the sum of all wisdom. Both must move ever onward in life, to and fro, like breathing in and breathing out. Whoever makes it a rule to test action by thought, thought by action, cannot falter, and if he does, will soon find his way back to the right road.
Wisdom, Goethe suggests, is not something the mind holds alone but a rhythm between mind and hands. Thought without action drifts into daydreaming, and action without thought hardens into habit or blind busyness, so each keeps the other honest. His image of breathing fits: you cannot only inhale or only exhale, and the back-and-forth is itself the healthy state. The promise at the end is modest and kind. We will still stumble, but someone who keeps testing ideas against experience, and experience against ideas, will notice the error early and find the road again.
This matches Goethe's general temperament. He distrusted pure abstraction and valued observation, practical work and lived experience, and he was a poet who also did hands-on natural science and administrative work. The line is usually traced to his late novel Wilhelm Meister's Journeyman Years, a book about learning through work and travel.
Today, someone unsure about changing careers might neither brood forever nor quit on impulse. They could take a short evening course or spend a day shadowing someone in the field, then write down what energized or bored them, adjust their view, and try the next small step.