If knowledge brings unhappiness it is better to be ignorant. The proverb comes from Thomas Gray’s Ode an a Distant Prospect of Eton College. The poet sadly considers the future of the boys there:
Alas! Regardless of their doom
The little victims play !
No sense have they of ills to come
Nor care beyond the day.
The last six lines of the poem run:
Yet, ah! Why should they know their fate,
Since sorrow never comes too late,
And happiness too swiftly flies?
Thought would destroy their paradise!
No more; – where ignorance is bliss,
‘Tis folly to be wise.