 Newton’s line of argument quoted above became one of the centerpieces of the debate that his paper generated. In some parts of his paper, when Newton wrote of the “rays” of light, he had evidently intended to remain neutral on the question of whether the rays are particles or waves (this is reminiscent of the ancient Greek practice of avoiding physical discussions of visual rays). But then towards the paper’s end, Newton added his new line of argument, which employed some philosophical analysis together with some experimental evidence to support the conclusion that rays of light cannot be waves after all. Newton’s critics pounced. This led to the first problem he encountered in response to his paper: what he calls his “theory” of light and colors was not merely rejected, but rather immediately misunderstood, at least from his own perspective. Just days after Newton’s paper was read at the Royal Society, Robert Hooke responded with a detailed letter to Oldenburg. In the first few sentences, Hooke indicates that from his point of view, Newton’s “Hypothesis of saving the phenomena of colours” essentially involves the contention that rays of light are particulate, rather than  wavelike.[8]  Hooke argues, in contrast, that light “is nothing but a pulse or motion propagated through an homogeneous, uniform and transparent medium;” that is, he argues that light is indeed wavelike. He makes it perfectly clear, moreover, that his hypothesis—the name did not carry a negative connotation in his work—can save the phenomena of colors just as well as Newton’s, which is to say, his hypothesis is compatible with the experimental evidence Newton had gathered. Evidently, the line of argument in the passage quoted above caught Hooke’s eye. Among philosophers, he was not alone. In a letter to Huygens explaining Newton’s theory of light, Leibniz writes that Newton takes light to be a “body” propelled from the sun to the earth which, according to Leibniz, Newton takes to explain both the differential refrangibility of rays of light and the phenomena of  colors.[9]  Since Newton had employed the concepts of substance, quality and sensible quality when concluding in his paper that light is (presumably) particulate, we are apt to regard the paper as contributing to important discussions within philosophy. After the extensive correspondence, and controversy, generated in response to Newton’s early optical views and experiments, he often threatened to avoid engaging in mathematical and philosophical disputes altogether. He insisted to friends and colleagues that he found intellectual controversy unbearable. But he never followed through with his threat to disengage from discussions in natural philosophy, sending many important letters throughout his long intellectual career.