The logical result of opposition is invention or adaptation. “Invention is a question followed by an answer.”[XXII-26] Invention, or adaptation, at its best is “the felicitous interference of two imitations, occurring first in one single mind.”[XXII-27] Inventions grow in two ways: (1) in extension--by imitative diffusion; and (2) in comprehension--by a series of logical combinations, such as the combination of the wheel and the horse in the inventions of the horse-cart.[XXII-28] Inventions partially determine the nature of new inventions and new discoveries. A new invention makes possible other inventions, and so on. Each invention is the possible parent of a thousand offspring inventions. To be inventive, one must be wide-awake, inquiring, incredulous, not docile and dreamy, or living in a social sleep. The inventor is one who escapes, for the time being, from his social surroundings.[XXII-29] Inventing develops from wanting. A man experiences some want, and in order to satisfy this want he invents. Inventiveness is contrary in nature to sheepishness. Since an invention is the answer to a problem, inventions are the real objective factors which mark the stage of progress. But invention, according to Tarde, becomes increasingly difficult. Problems naturally grow increasingly complex as the simpler ones are mastered. Unfortunately, the mind of man is not capable of indefinite development, and therefore will reach a limit in solving problems.[XXII-30] At this point, Tarde is on doubtful ground. His argument can neither be proved nor disproved. Apparently, man’s ability to solve problems increases with his training and experience in that connection. Moreover, man appears to be at the very dawn of his possibilities in the field of invention. He is only beginning to gather together systematically the materials for inventing, and to understand slightly the principles of inventing. Inventors are imitative.[XXII-31] This statement is but another way of saying that inventions are cumulative, that they come in droves, that they are gregarious. A new discovery will arouse the ambition of many wide-awake persons to make similar discoveries. “There is in every period a current of inventions which is in a certain general sense religious or architectural or sculptural or musical or philosophical.”[XXII-32] Invention and imitation represent the chief forces in society.[XXII-33] Invention is “intermittent, rare, and eruptive only at certain infrequent intervals.” It explains “the source of privileges, monopolies, and aristocratic inequalities.” Imitation, on the other hand, is democratic, leveling, and “incessant like the stream deposition of the Nile or Euphrates.” At times the eruptions of invention take place faster than they can be imitated. At other times imitations flow in a monotonous circular current.