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.