In his Narrative of the Life of an American Slave,
Frederick Douglass argues [present tense] that slavery
corrupts [present tense] the slaveowner.
This convention is also used when discussing works of art. "Painted in the same year as Nighthawks, Dawn in Pennslyvania is also dominated [present tense] by an isosceles trapezoid . . ." (Hopper, Mark Strand). If the text you are discussing is a history, you may need to refer to historical fact in past tense while sticking consistently to present tense when referring to the text itself.
Because he had experienced [past perfect tense] slavery
first hand, Douglass knew [past tense] that owning slaves
could transform otherwise good people into virtual
monsters. Thus, he argues [present tense] that slavery corrupts
[present tense] the slaveowner.
Scientific fact is also reported in the present tense.
Newton discovered [past tense] that white light is composed [present tense] of wavelengths of various colors.
As are general truths.
Freud was [past tense] not the first to conclude that people do [present tense] not always behave rationally.
When instructors tell you to avoid shifting tense, they don't mean that you never change tense. What they mean is that shifts should be logical and that you should employ the convention of literary present consistently.