One main objection to presentism comes from the idea that what is true substantively depends upon what exists (or, that truth depends or ' supervenes ' upon being). In particular, presentism is said to be in conflict with truth-maker theory, one theory which looks to capture the dependence of truth upon being with the idea that truths (e.g., true propositions) are true in virtue of the existence of some entity or entities ('truth-makers'). The conflict arises because most presentists accept that there are evidence-transcendent and objective truths about the past (and some accept that there are truths about the future, pace concerns about fatalism), but presentists deny the existence of the obvious truth-makers for such truths. For instance, most presentists accept that it is true that Marie Curie discovered polonium, but they deny that the event of her discovery exists (because it is a wholly past event). Since the mid-1990s, Truth-Maker theorists are trying to accuse Presentists with violating the plausible principle (that truths require truth-makers) and ontologically 'cheating'. Presentists can respond to this objection either by denying that truths about the past require truth-makers (that is, they can accept the truth-maker principle for some truths, but deny that it applies in full generality, or else reject the truth-maker principle wholesale), or by locating presently existing entities to play the role of truth-makers for truths about the past.