Ignorance is bliss but . . . .?
From Thomas Gray's poem, Ode on a Distant Prospect of Eton College (1742)
the actual full phrase...
"—where ignorance is bliss, 'Tis folly to be wise." (Thomas Grey Archive)
Many compare this with the phrase: "What you don't know can't hurt you"
I prefer Ricky Gervais' take:
"Ignorance is bliss. But only for the ignorant."