Classical models studying the evolution of self-fertilization in plants conclude that only complete selfing and complete outcrossing are evolutionarily stable. In contrast with this prediction, 42% of seed-plant species are reported to have rates of self-fertilization between 0.2 and 0.8. We propose that many previous models fail to predict intermediate selfing rates because they do not allow for functional relationships among three components of reproductive fitness: self-fertilized ovules, outcrossed ovules, and ovules sired by successful pollen export. Because the optimal design for fertility components may differ, conflicts among the alternative pathways to fitness are possible, and the greatest fertility may be achieved with some self-fertilization. Here we develop and analyze a model to predict optimal selfing rates that includes a range of possible relationships among the three components of reproductive fitness, as well as the effects of evolving inbreeding depression caused by deleterious mutations and of selection on total seed number. We demonstrate that intermediate selfing is optimal for a wide variety of relationships among fitness components and that inbreeding depression is not a good predictor of selfing-rate evolution. Functional relationships subsume the myriad effects of individual plant traits and thus offer a more general and simpler perspective on mating system evolution.