Someone might object to the use of feeling as a guide to knowledge. But a simple consideration will show that in fact this is what feeling does. Anyone who takes knowledge seriously will admit that in acquiring knowledge we must proceed logically. We use logic as an instrument for testing the knowledge we acquire. How, then, if logic is the instrument, can logic itself be proved? One might say that logic can prove itself. Yes, but before we begin proving logic by logic, it must be at least possible to grasp logic with our feeling. Logical thought cannot be proved primarily by logical thought, but only by feeling. Indeed, everything that constitutes logic is first proved by feeling, by the infallible feeling for truth that dwells in the human soul. From this classic example we can see how feeling is the foundation of logic and of thinking. Feeling must give the impulse for the verification of thought.
What must feeling become if it is to provide an impulse not only for thinking in general, but thinking about worlds we are at first unacquainted with and cannot survey? Feeling of this kind must be a force that strives from within toward an object yet unknown. When the human soul seeks to encompass some other thing with feeling, we call this feeling love. Love can of course be felt for something known, and there are many things in the world for us to love. But as love is a feeling, and as feeling is the foundation of thinking in the wider sense, we must be clear that the unknown supersensible can be grasped by feeling before thinking comes in. Accordingly, unprejudiced observation shows that it must be possible for human beings to come to love the unknown supersensible before they are able to conceive it in terms of thought. This love is indeed indispensable before the supersensible can be penetrated by the light of thought.
Source: "The Mission of Reverence" — lecture of October 28, 1909; from Love and Its Meaning in the World, pp.48-49