No physicist has ever "seen" an electron. Yet, all physicists believe in the existence of electrons. An intelligent but superstitious man advances this analogy to argue that "ghosts" exist even though no one has ëseení one. How will you refute his argument ?
NCERT Solutions for Class 11 Physics Part 1 Chapter 1 Exercise 5



Yes it is true that no physicist has ever seen atom but its existence has been established through so many indirect experimental  evidences.
 on the other hand there is no direct or indirect evidence which may establish the existence of ghosts