If you aren't referring to specific events, but just to changes in public opinion or something equally intangible, make sure to give some sort of time-line within the structure of your paper. The quotes and references you choose should be carefully selected so that they prop up your argument without making it appear as though your idea is so tentative that you feel you must bombard the reader with proof that you're right. If you cannot be sure whether an assertion in students' essays come from the students themselves or from the philosopher they are discussing, ask them to make that clear in the essay. The writers of philosophical texts have often already noticed what might be considered holes in their arguments and provided answers to possible objections. This is one reason why philosophical texts must be read with utmost care.
Students often let quotations from philosophical texts speak for themselves.
If you find that students writing a philosophy paper have included quotations but have not explained them in their own words or explained how they fit into what they are trying to say in the essay, ask them to explain the quotation in their own words and explain how it fits into the essay. Then ask them to include something like what they just said in the essay. If students write an essay comparing the ideas of two different philosophers on a certain subject, ask them to include explanations of how each philosopher defines key terms. Exactitude in the definitions of key terms is essential in philosophy. Ideally, a philosophy paper is supposed to have a clear thesis statement in the last sentence of the first paragraph, and the first sentence of each succeeding paragraph should refer back to this thesis.