"What a Fool Believes" by the Doobie Brothers was number one on the Billboard pop chart 40 years ago.
The song was co-authored by lead singer Michael McDonald and Kenny Loggins. Loggins also recorded the song for one of his solo albums.
The Doobie Brothers' version won Grammy awards for Song of the Year and Record of the Year. It was one of just a few non-disco tunes to top the charts in the first eight months of 1979.