  Generally when a sign appears as part of a declarative sentence, it is the reference of the sign that is important. Both “Venus” and “the morning star” designate the same object. The sentence “The morning star is seen in the sky near sunrise” is true, and remains true when “the morning star” is replaced with “Venus”. Substitution of equi-designating signs preserves truth. But not always; there are contexts in which this does not happen, indirect reference contexts. As a typical example, “George knows that the morning star is seen in the sky near sunrise” may be true while “George knows that Venus is seen in the sky near sunrise” may be false. Besides knowledge contexts, indirect reference arises when a sentence involves “I believe that…”, “I think that…”, “It seems to me that…”, “It is surprising that…”, “It is trivial that…”, and so on. In such contexts, Frege concludes, not designation but sense is central. Then, since “George knows that…” is an indirect reference context, senses are significant. The signs “the morning star” and “Venus” have different senses, we are not replacing a sense by a sense equal to it, and so should not expect truth to be preserved.