Such a choice, in other words, determines to some extent the sort of elements that will most probably be selected to make up at any moment the contents of your consciousness. [Sidenote: _Iron Filings and Mental Magnets_] From the instant that you make such a choice you are on the alert for facts relevant to the subject of your ambition. Upon them you concentrate your attention. They are presented to your consciousness with greater precision and clearness than other facts. All facts that pertain to the art of flying henceforth cluster and cling to your conscious memory like iron filings to a magnet. All that are impertinent to this main pursuit are dissociated from these intensely active complexes, and in time fade into subconscious forgetfulness. [Sidenote: _The Compartment of Subconscious Forgetfulness_] By subconscious forgetfulness we mean a _compartment_, as it were, of that reservoir in which all past experiences are stored. _Consciousness is a momentary thing._ It is a passing state. It is ephemeral and flitting. It is made up _in part of present sense-impressions_ and in part of past experiences. These past experiences are brought forth from subconsciousness. Some are voluntarily brought forth. Some present themselves without our conscious volition, but by the operation of the laws of association and dissociation. Some we seem unable voluntarily to recall, yet they may appear when least we are expecting them. It is these last to which we have referred as lost in subconscious forgetfulness. As a matter of fact, _none_ are ever actually _lost_. [Sidenote: _Making Experience Count_] All the wealth of your past experience is still yours--a concrete part of your personality. All that is required to make it available for your present use is a sufficient concentration of your attention, _a concentration of attention that shall dwell persistently and exclusively upon those associations that bear upon the fact desired_. The tendency of the mind toward dissociation, a function limiting the indiscriminate recall of associated "groups," is also manifested in all of us in the transfer to unconsciousness of many _muscular activities_. [Sidenote: _How Habits Are Formed_] As infants we learn to walk only by giving to every movement of the limbs the most deliberate conscious attention. Yet, in time, the complicated co-operation of muscular movements involved in walking becomes involuntary and unconscious, so that we are no longer even aware of them. It is the same with reading, writing, playing upon musical instruments, the manipulation of all sorts of mechanical devices, the thousand and one other muscular activities that become what we call _habitual_. The moment one tries to make these habitual activities again dependent on the conscious will he encounters difficulties.