In this thesis our daily held beliefs about what would be an (un)reasonable moral demand are discussed. In a philosophical context, our commonsense beliefs about the limits that we should respect to the obligations and duties that can be imposed on moral agents, are not easily defended, even though they are widely held by many. My thesis is that these commonsense ideas have not been done justice so far in the philosophical debate, and I therefore concentrate on an analysis of the content of our ideas about reasonable demands, which I formulate as a Reasonableness Claim. I argue that the belief that some burdens will be unreasonable from an agent's perspective is at least supported in four different ways by commonsense morality. This makes it more plausible to accept that 'cost to the agent' is a serious factor we have to account for in normative theory. I argue also that the widespread belief that agents should have sufficient opportunities to pursue their own interests is related to notions that matter to people: their autonomy, their integrity and the importance of caring relations, and not, as critics argue, to their egoistic interests only. In different degrees and different contexts, the claim that one's personal interests could prevail other considerations, is supported and made more plausible in the 'debate on the limits of morality'. The Reasonableness Claim, I argue, supports the view of a human morality, that enables agents to choose and balance different, but equally important considerations as seen from their perspective.