(1) We shouldn’t be cruel to animals, i.e. we shouldn’t harm animals unnecessarily.
(2) The consumption of animal products harms animals.
(3) The consumption of animal products is unnecessary.
(4) We shouldn’t consume animal products.
An obvious point of attack would be the notion of necessity.
a) It seems plausible that much if not most of the third world might not currently have sufficient access to an adequately nourishing vegan diet. The above argument would not seem to apply to such populations, other than perhaps in an aspirational sense; by suggesting it might be valuable/a moral obligation to enable such countries to reach a point at which consuming animal products becomes unnecessary.
b) 'Necessary' might encompass concepts other than nutrition/the sustenance of human life. For some people, the taste pleasure of meat and/or dairy may be deemed 'necessary'. Another person might deem that the pleasure of killing itself (such as via the hunt) might be deemed necessary, or that the hunt constitutes a sacred practice. However, the acceptance of such views might lead to a situation under which any subjective notion of necessity might be sufficient to render cruelty acceptable, including towards humans.
Are there any philosophers/philosophical works which address subjective notions of ethical/moral necessity, including perhaps how we decide what necessity entails? Does libertarianism deal with this topic in any detail? I'm particularly interested in any who explore this area in relation to animal welfare and in any who present robust philosophical justifications for speciesism, and more specifically for the cruelty we routinely inflict on animals for the purpose of obtaining animal products.
A determinist might state that our attitudes and actions are inevitable and that therefore so is cruelty; that cruelty is in a way 'necessary' as the product of all prior circumstances to date. This view (whilst persuasive) is perhaps of little value to the intricacies of this particular ethical discussion however or, (if it is wrong), might lead to inaction where action might in fact be the result of true agency.
Evolutionary psychologists might also have insight; I suspect not too dissimilar from that of determinists, in the sense that our lingering desire to eat meat and drink milk and wear skin is inevitable given the length of time for which these things have been necessary. More interesting however than the inevitability of such desire - and more pertinent to this stack - is why, given unnecessary cruelty to one another as humans is typically deemed undesirable, 'necessary' cruelty to animals might be afforded a lower threshold.
'Are there any philosophical positions which seek to justify speciesism, in particular as it pertains to the 'unnecessary' cruelty we inflict on other animals?.
(Wikipedia summarises some arguments for and against speciesism - including those of Cohen, Williams, Staudenmaier, Scruton, Peikoff and Noddings - but these 'for' arguments don't tend to grapple with the 'unnecessary' aspect of human cruelty to animals, and the 'unnecessary' component of Effective Altruism's argument contributes greatly to its weight).