"After these common mercies, I say, whereof the reprobate are often partakers, he openeth the treasure of his rich mercies, which are kept in Christ Jesus for his Elect ... Such as willingly delight not in blindness may clearly see that the Holy Ghost maketh a plain difference betwixt the graces and mercies which are common to all, and that sovereign mercy which is immutably reserved to the chosen children..."
"On Predestination," in The Works of John Knox, ed. David Laing, (Edinburgh: Wodrow Society, Edinburgh, 1856), 5:87.
If John Knox calls men blind who can't see the difference between 1) the grace/mercy which is common to all and 2) the sovereign grace/mercy that is reserved for the elect alone, then surely he would call those blind who can't even see the existence of such a thing as common grace.