From the "Faithful and Wise Servant":
All truth is in harmony, whether old or new. What was true
at any time in the past must be true yet....
and if it
is not true now, instead of being advanced light
then it was terrible darkness.
[...] We have sometimes
been accused by unbelievers for teaching that the
true way to advance was to displace the truth we
learned yesterday by new truth learned today; but
we utterly repudiate the absurd charge. To grow in
knowledge is to retain the truth we have and add
to our stock.
(Zion’s Watch Tower July 1880 WTR p. 119)
If we
were following a man undoubtedly it would be different
with us; undoubtedly one human idea would
contradict another and that which was light one or
two or six years ago would be regarded as darkness
now: But with God there is no variableness,
neither shadow of turning, and so it is with
truth; any knowledge or light coming from God must
be like its author. A new view of truth never can
contradict a former truth. "New light" never
extinguishes older "light," but adds to it. If you were
lighting up a building containing seven gas jets
you would not extinguish one every time you
lighted another, but would add one light to another
and they would be in harmony and thus give increase
of light: So is it with the light of truth; the true
increase is by adding to, not by substituting one for
another.
(Zion's Watch Tower February 1881 WTR p.
188)