ABSTRACT In this essay, we aim at framing the ‘negative emotion’ of indifference, starting from its diachronic declination, which seems to beneficiate from a form of justification from the moral point of view (§1). In order to prevent indifference as an outcome – together with its intrinsic motivational strength –, we introduce a methodological account to frame the struggle of motivation internal to the single agent, by classifying different forms of ‘reasons to act’ (§2). We will develop a two-move strategy. Firstly, we deploy what we could call a positive emotion – the sense of solidarity diachronically understood – against that negative one, in order to show that indifference is not the sole possible destination for the humankind (§3). Secondly, to integrate and strengthen the motivational role of that positive emotion, we will rehabilitate a moral approach aimed at setting up a unique normative linkage among generations, by reshaping the interplay between ‘wide’ and ‘narrow obligations’ presented by Kant (§4). The ultimate goal is to contrast the moral strength which is offered by indifference as negative emotion by articulating a motivational path devoted at legitimizing the diachronic moral commitment and duties of justice among generations.