In this work-in-progress, I propose that a primitive distinction between metaphysical and epistemic bases for reasoning provides a unified account for a range of constraints on the occurrence and interpretation of tense-aspect morphology in indicative conditionals. The main data I discuss is from Bangla, where two types of conditional constructions-- canonical if-conditionals and conditional-participial sentences-- systematically restrict the temporal properties of the antecedent. I then present preliminary data from a set of non-if conditional constructions in English that pattern similarly. I propose that what unifies these constraints is a strong preference for the proposition in the antecedent to be metaphysically open. This derives from a cross-linguistically general tendency for indicative conditionals to be asserted under uncertainty about the truth of their antecedent. I suggest that different flavors of modality (epistemic/subjective vs metaphysical/objective) create an entailment scale of uncertainty, with certain devices for conditional expression across languages invoking the stricter end of this scale. This requirement for "metaphysical uncertainty" interacts with the semantics of tense-aspect and the asymmetry between a 'fixed' past and an 'open' future to generate the observed restrictions. This brings up open questions about the appropriate semantics for the relevant aspectual categories, particularly the progressive. The analysis is implemented in a branching time framework, which represents the past-future asymmetry formally. I end with a prospective discussion about the possible status of these restrictions in the grammar, and the broader relevance of a metaphysical-epistemic distinction as it relates to modal expressions in natural language, particularly conditionals.