Is it a valid phrase? Please, accept or reject it. Maybe there is other exact expression.
closed as off topic by simchona♦, Jasper Loy, Thursagen, FumbleFingers, kiamlaluno Sep 7 '11 at 11:16
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Your phrase "to live in ignorance of just happiness" is grammatical but doesn't sound quite right. "ignorance of X" means not knowing about X, but I don't think you mean "not knowing about just happiness".
Perhaps you are looking for the well-known English saying "Ignorance is bliss" and the corresponding idiomatic phrase "blissful ignorance". For example, "I'd rather not tell my children about the famine in Somalia; they will be happier to live in blissful ignorance."