"I myself am pursuing the same instinctive course as the veriest human animal you can think of I am, however young, writing at random straining at particles of light in the midst of a great darkness without knowing the bearing of any one assertion, of any one opinion. Yet may I not in this be free from sin?"

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All our thoughts and concepts are called up by sense-experiences and have a meaning only in reference to these sense-experiences. On the other hand, however, they are products of the spontaneous activity of our minds; they are thus in no wise logical consequences of the contents of these sense-experiences. If, therefore, we wish to grasp the essence of a complex of abstract notions we must for the one part investigate the mutual relationships between the concepts and the assertions made about them; for the other, we must investigate how they are related to the experiences.

— Albert Einstein