"Never believe a thing simply because you want it to be true"
A warning against wishful thinking attributed to Diax, who once chased fortune tellers away from a temple with a garden rake before cautioning his colleagues with the words that would go on to form the Rake.
This may be a reference to Thucydides, who said, "…[I]t is a habit of mankind to entrust to careless hope what they long for, and to use sovereign reason to thrust aside what they do not desire."
Another possibility is Francis Bacon: "The human understanding when it has once adopted an opinion ... draws all things else to support and agree with it. And though there be a greater number and weight of instances to be found on the other side, yet these it either neglects or despises, or else by some distinction sets aside or rejects[.]" 
- "…[I]t is a habit of mankind to entrust to careless hope what they long for, and to use sovereign reason to thrust aside what they do not desire." 
- Bacon, Francis (1620). Novum Organum. reprinted in Template:Citation via Template:Harvnb