Pavlov looked at the balance between excitation and inhibition and determined that there were different types of nervous systems. These observations were made about dogs, but Pavlov believed that they could be generalized to humans as well. The different types of nervous systems were categorized by Pavlov into strong and weak nervous systems with different sub-types under the strong type. These different types of systems were categorized based on the strength of excitation or inhibition in the system of the observed individual. Those individuals with weak nervous systems were thought by Pavlov "to have limited adaptive abilities," while strong nervous systems were more adaptable.