I is for Intangible

intangible (adjective): something that exists but that cannot be touched, exactly described, or given an exact value. 

This may mean:

• something that is unable to be physically touched: (immaterial, incorporeal, impalpable, insubstantial, ethereal, ghostly)
• something that is difficult to perceive or grasp: (elusive, impalpable, imperceptible, vague.)
• something that is abstract or theoretical: (hypothetical)

G is for Ghost (Randall & Hopkirk)

"Only You Jeff, Only You"

'Randall and Hopkirk (Deceased)' was a late 1960s British TV series which starred Mike Pratt and Kenneth Cope as the private detectives Jeff Randall and Marty Hopkirk. The series explored new ground as the show's first episode saw Hopkirk murdered in the line of duty, returning as a white-suited ghost that only Randall was able to see or hear. The show was one of the ITC shows that dominated Sixties' TV in the UK, and was one of the better offerings. Only one series of 26 episodes was shot, however the show was repeated many times. A remake was made in 2000 starring British comedy duo Vic Reeves and Bob Mortimer, but it's the original show I remember most fondly.