As noted in the beginning, touch seems to have both a passive and an active nature. In its more passive forms, touch involves cutaneous activations across the surface of the body. These include bodily sensations of hot and cold, pressure, vibration, and the like. In addition, these activations have a limited, entirely bodily spatial character. This form of touch has played an important role in the early empirical study of touch, especially in determining the nature and acuity of our sensory receptor populations. This has often been accomplished through the measure of the two–point threshold mentioned above. This distance varies across the body and is a reliable guide to the density and spatial resolution of many tactual receptors on the   body.[4]  While these passive forms of touch are important, especially in the empirical investigation of touch, in typical, unconstrained touch, we use every part of our bodies to actively explore the environment. We use our hands and arms and fingers, we move our trunks and legs, and we actively feel with nearly every surface of our body. Touch in ordinary use involves specialized movements and grips. Such prehensile manipulation through touch is often distinguished from stereognosis, or object recognition through touch. In addition, these movements recruit and make use of many receptive systems that seem inherently active. For instance, haptic touch involves feedback from our movements, along with information from our motor activities (both in motor planning and efference copies). Perhaps most importantly, it also includes information from receptors in our muscles and joints. All of these elements interact and play a critical role in forming and developing our sense of touch. For this reason, touch, especially of the active, haptic variety, seems like an ideal model for views of perception according to which perception is essentially a form of action, or at least a form of experience that involves action in a unique manner.