Dewey goes on to say that care must be taken in the use made of these sign-functions in connection with inference. "There is a great difference between saying that the perception of a shape affords an indication of how to act and saying that the perception of shape is itself an inference."[267] No judgment, Dewey seems to imply, is involved in responding to the motor cue furnished by a familiar object. Again, the common idea that present perception consists of sensations as immediate, plus inferred images, implies that every perception involves inference. But the merging of sensations and images in perception can be explained naturally, by the fusion of nervous processes, and no supplementary (transcendental) act of mind is needed to explain the integrity of experience. The tendency to take perception as the object of knowledge, Dewey continues, instead of as simply cognitive, a term in knowledge, is due to two chief causes. The first is that in practical judgments the pointing of the thing towards action is so universal a trait as to be overlooked, and the second is that signs, because of their importance, become objects of study on their own account, and in this condition cease to function directly as cognitive. Dewey means, apparently, that because the cognitive aspect of things is never attended to except when they are 'known,' or treated as objects of judgment, there is a tendency to suppose that they always have the character that pertains to them as 'known' things. Again, Dewey says, perception may be translated as the effect of a cause that produced it. But the cause does not ordinarily appear in experience, and the perceptions, as effects, remain isolated from the system of things. Truth and error then become matters of the relation of the perception to its cause. The difficulties attendant upon this view can be avoided by taking sense perceptions as terms in practical judgments. Here the 'other term' which is sought is the action proposed by the perception. "To borrow an illustration of Professor Woodbridge's: A certain sound indicates to the mother that her baby needs attention. If there is error it is not because the sound ought to mean so many vibrations of the air, while as matter of fact it doesn't even suggest air vibrations, but because there is wrong inference as to the act to be performed."[268] The idea is tested, not by its correspondence with some formal reality, but by its ability to lead up to the experience to which it points.