Dear @Hollytibble, I think you're right, there's essentially nothing that makes any choice over any other DISCRIMINATION. For example, a manager may make their mind up that they want to hire a person of a certain gender or looks (for example a bald person) for a given job and then hide their choice behind the rejection letters which will read 'We're sorry it was very competitive ..." and so on. For the smokers, I think I'm not sure theres lower productivity and more time off, it depends on the kind of job you want them for. For example, if your smoker is a candidate for an academic job or a construction worker, it only matters if you impose harsh and unfair metrics on them, such as count the minutes they waste or try to blame them any time they get sick leave - which they're entitled to. So I don't know if the productivity and time off are moral reasons to not hire somebody, in the sense that you may be impeding on their rights. It seems to me that the example you give about horse psychotherapy and coding is the most clearcut, though who knows, one may be able to come up with a reason why that's discrimination too (for example evidence of lack of human communication skills because of the preference to horses). So to sum up my challenge: I think that there is no line, what makes something DISCRIMINATION (with capital letters) is quite arbitrary, as are the various LAWS which regulate it - they're the product of negotiation and of the power-play between different interests within parliaments (and maybe society), with sometimes the added disadvantage that they are quite anacronistic due to them (the LAWS) not changing as society's dominant ethics shift. I wish the above satisfies the PHILOSOPHICAL condition :)