Discovery
Over time, humanity invented objects and methods for accomplishing tasks which fulfill some purpose in a new or different manner, usually with the objective of realizing that purpose in a faster, more efficient, easier or cheaper way.Although a new or useful object or method may be developed to fulfill a specific purpose, the original idea may never be fully realised as a working invention, perhaps because the concept is in some way unrealistic or impractical.
A "castle in the air" or a "pie in the sky" (or "castles in Spain") may refer to a creative idea which does not reach fruition due to practical considerations. The history of invention is full of such castles, because inventions are not necessarily invented in the order that is most useful; for example, the design of the parachute was worked out before the invention of powered flight. Other inventions simply solve problems for which there is no economic incentive to provide a solution.
On the other hand, any barriers to implementation may simply be an issue of engineering or technology which can be overcome in time with scientific advances. History is also replete with examples of ideas which have taken some time to reach physical reality, as demonstrated by various ideas originally attributed to Leonardo da Vinci which are now expressed in everyday physical form.