Angel notices a snake slithering up the river bank while he is fishing. The thalamus of Angel's brain immediately stimulates his autonomic nervous system, which, in turn, increases the rate of his breathing and heartbeat. At the same time, Angel's brain sends signals to his cerebral cortex which causes Angel to perceive the emotional experience of fear. Consequently, Angel experiences both fear and the physiological changes in his body at the same time. This scenario complies with the principle of the _____.