The phrase ignorance is bliss means that, if one is unaware of an unpleasant fact or situation, one cannot be troubled by it.

This phrase was coined by the English poet and literary scholar Thomas Gray (1716-1771) in An Ode on a Distant Prospect of Eton College (London: printed for R. Dodsley and sold by M. Cooper, 1747):

To each his Suff’rings: all are Men,
Condemn’d alike to groan,
The Tender for another’s Pain;
Th’ Unfeeling for his own.
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.

These are the earliest occurrences of the phrase that I have found, in chronological order—apart from the citations of, and the allusions to, An Ode on a Distant Prospect of Eton College:

1-: From An Epistle to a Friend, from St Andrew’s. Lately found among some old papers, a poem published in The Scots Magazine (Edinburgh, Midlothian, Scotland) of May 1765:

’Twas peace, ’twas ease, ’twas love, ’twas happiness;
Where is that bliss, that happiness refin’d,
That chain’d, inchanted, rivetted, the mind?
As fair the narciss in the garden blows,
As smooth the stream of silver Eden flows,
As sweet the thrush repeats her tender tale,
As soft the zephyr travels o’er the vale,
As bright the hallow’d hand of young-ey’d Spring,
The lucid dewdrops o’er the fields does fling;
Yet peace, yet bliss, yet love, is wanting here;
And discontent still drops th’ unwilling tear.
Oh! can it be that ignorance is bliss?

2-: From The Social Fire, a poem published in The British Chronicle. Or, Pugh’s Hereford Journal (Hereford, Herefordshire, England) of Thursday 23^rd March 1786:

Oh! grant kind Heav’n a state like this,
Where simple ignorance is bliss,
’Tis all that I require;
Then, then—to share the joys of life,
I’d seek a kind indulgent wife,
And bless my Social Fire.

3-: From Volume III of The Rector’s Son (London: Printed for Lee and Hurst, 1798), a novel by the English author and translator Anne Plumptre (1760-1818):

“Villainy, too prosperous villainy, has led them into a fatal delusion; nor are they less deserving of compassion than myself!—but they are less miserable, for they know not the occasion they have for remorse;—to them, ignorance is bliss; to me, knowledge is distraction!”