imagination

• Have your head in the clouds

  To be lost in daydreams, impractical, or not paying attention to what is happening around you.

• Pipe dream

  An unrealistic, unattainable, or impossible hope, plan, or fantasy.

• Mind's eye

  The mind's eye is the faculty of the imagination, enabling one to visualize or remember things that are not physically present.

• Build castles in the air

  To dream or plan fanciful, unrealistic schemes that are unlikely to ever come true.

• A flight of fancy

  An imaginative idea or thought that is whimsical, unrealistic, or impractical.