 Sentences like ‘Kepler thought that the Earth was spherical’ embed other sentences — ‘the earth was spherical’ — in oblique (or opaque or intensional (with an ‘s’)) contexts. Oblique contexts are not truth-functional. Substituting other true sentences for ‘the earth was spherical’ will not necessarily preserve truth. Moreover, even substituting co-designative terms for ‘the earth’ may not preserve truth. Kepler did not, one supposes, think that Frege’s favorite planet was spherical (assuming the earth was Frege’s favorite planet). 