Co-written and first recorded by Kenny Loggins (1978).
Hit version by The Doobie Brothers (US #1 1978).
From the wiki: “‘What a Fool Believes’ was written by Michael McDonald and Kenny Loggins. The best-known version was recorded by The Doobie Brothers (for the album Minute by Minute), with McDonald on lead vocal, but the song was first recorded and released five months prior by Loggins on his album Nightwatch. McDonald’s original idea for the song was that of a man who is reunited with an old love interest and attempts to rekindle a romantic relationship with her before discovering that a relationship had never really existed. It was Loggins, right before meeting McDonald for the first time, who devised the bridge of the song.
“According to Loggins, the two were introduced by a mutual friend but had never before met. While walking up to the door of McDonald’s home for their first meeting, Loggins heard McDonald playing the melody (no lyrics) on a piano when McDonald stopped at the bridge. Loggins’ mind continued without a break … and the song’s bridge was born. Then, Loggins knocked on the door, introduced himself to McDonald and, before the two could sit down, demonstrated the bridge. The lyrics were finished over the telephone the next day. Each writer participated in the other’s respective recordings of the song.
“‘What a Fool Believes’ was one of the few non-disco #1 hits on the Billboard Hot 100 during 1979. The song received Grammy Awards in 1980 for both Song of the Year and Record of the Year.”
The Doobie Brothers, “What a Fool Believes” (1978):
Michael McDonald & Kenny Loggins, “What a Fool Believes” from Outside: From the Redwoods (1993):