Thus when we say, for example, that the crystalline lens in the eye has the purpose of focusing, through a secondary refraction, the light rays emanating from a certain point into a point on the retina, we are merely saying that we think the representation of a purpose in the causal action of nature in producing the eye because such an idea serves as a principle for conducting our investigation concerning this part of the eye, and thus also assists us to devise possible means of enhancing the relevant effect.