The schoolboy writes these figures on his slate: 2+2=4, and
says two and two make four. But the two and the two which he
has written on the slate do not make the four which he has
written on the slate. For both the twos are there unchanged,
and the four also. The two and two that make four are in his
head--invisible. The figures on the slate are not the realities,
they are only symbols which interpret the realities, and the
realities are invisible--LYMAN ABBOTT, _The Outlook_.
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