Reality exists in the material object, not in the immaterial mind. Therefore, it is the mind that must conform to the object - not the object to the mind. This conformity of the mind to the object is done thru the senses and is an entirely physical process. Science is thus a primary concern for the Naturalist. For the Naturalist, the baseline of value is that which is natural - that is, that which is in conformity with nature.
Nature is good.
One need not look beyond nature to some immaterial ideal for a standard of right and wrong. Rather, goodness will be found by living in harmony with nature. It is a breaking of the natural law. In seeking good appearance, or sound, the Naturalist will look to nature as the standard. Thus, for the Naturalist, art should imitate nature.