Strength of excitation was considered to be the most important of the nervous system properties by Pavlov because we are often confronted by stimuli in the environment that grab our attention immediately. Excitation of the cells is simply their ability to do work, and this can either be very short-lived or extended and have varying intensities. This excitation can lead to the gain of conditioned responses, or the generalization of a conditioned response. Inhibition, on the other hand, is when a response that is already encoded is overcome or contained. Mobility is the ability of the nervous system to give one impulse priority over another. This can be done by having excitation before inhibition, or vice versa.