Ought sexual activities, desires, pleasures to be assessed only according to general moral principles which apply to other areas of our lives? For example, is consent enough to justify our sexual choices? And if so, does that make sexual ethics different from other areas of ethics – or alternatively, does it make it the same as other areas, if we are inclined to think the rest of ethics is as ‘thin’ as that?

Enter Alexander Pruss, a philosopher well-schooled in both the analytic and phenomenological/personalist traditions, to tackle some of these questions. This profound and very readable work sets a new standard for sexual ethics. Built around its central argument is a profound examination of a wide variety of sexual phenomena, many of which are of pressing ethical concern yet are only cursorily dealt with by previous writers. Included are original discussions of sexual desire, the place of libido in human life, sexual fantasy, non-coital sexual activity, same-sex attraction, contraception and natural family planning as well as questions surrounding reproductive technology. Pruss's central argument begins with a discussion of love: a subject which, commendably, is not simply tagged on as an afterthought in an abstract origins argument about the purpose of sex. In discussing agape in its various forms Pruss notes that ‘each of [love's] forms is in some way the same, and yet the forms are different. Moreover, love becomes distorted when we get the form wrong – for instance, by standing in a relation of eros to one's parent’ (p. 7). Establishing convincingly that willing the good of the beloved for the beloved's sake, appreciating the beloved and seeking union are necessarily intertwined, Pruss notes that, unsurprisingly, love is experienced as one thing. Moreover, ‘Our love is humble insofar as it is a response to reality. The central salient part of that reality is the beloved. But that is not all. We also need to humbly, i.e. realistically, examine ourselves and our relationship with the other, and there is an objectivity here. The nature of love calls on us to respond to reality, and this need to respond to reality is what makes the duties of love not to be subjective…The other-focus of love then goes hand-in-hand with a rejection of a relativistic approach to ethics’ (p. 30).

Following Aquinas's notion of ‘formal union’ Pruss notes that one's will is united with the beloved's in willing the beloved's good. That said, there is a union that is had simply in virtue of loving. In recognising the beloved as a person, ‘one recognizes that the beloved has a point of view, and by recognizing the beloved as a human, one realizes what certain aspects of this point of view must be’ (pp.31–32). However, formal union, which is a necessary part of love, can impel us to ‘real union’ – a union which is a way of being together, not just in mind and will, but externally and cooperatively. And love does not seek just any old real union, but one expressive of the form of love in question. What primarily differentiates the forms of love is not the benevolent or appreciative aspects, not the formal union part of the unitive aspect, but the kind of real union that the relationship calls for. For Pruss it appears that some kinds of real union are paradigmatic and consummatory of a particular form of love.

The real union of the activity of sexual intercourse involves a set of organs functionally matched and striving towards a momentous (non-trivial) end. It involves a union as one flesh, one body where two bodies come together united by a common biological striving for reproduction. But two embodied persons are involved and they yearn for a union that does justice to a form of interpersonal love – one in which the biological aspect of the union needs to be in harmony with the attitudes of the two as persons. In short, the couple cannot fight that which unites them if this is to be an integrated real union of persons.

Pruss considers forms of love and focuses on specifically erotic love as one that seeks a real union of two persons as one organism in loving lifelong commitment through a personally integrated reproductive striving (lifelong, so the brief act of intercourse can be stretched through time by an act of commitment allowing for sexual union as a fully personal union of one body).

I cannot do justice here to the depth and detail of the arguments put forward by Pruss. Suffice it to say that he lays out precisely the meaningfulness of sex in human life, examining the question at many levels. Some sections are, admittedly, incomplete in feel, as with Pruss's interesting discussion of sexual desire and ‘doomed’ erotic love in cases where genuine erotic union is literally impossible (such as incestuous or same-sex erotic love): it would be good to hear more on the connection – sometimes a very obscure connection – between felt desires and underlying ‘desires’ or needs for friendship and more realistic union.

Helpfully distinguishing between desire for union with another and the more generic libido, Pruss further states that ‘libido implicitly points us toward union with another human being, both because it is reproduction with another that is desired – humans reproduce sexually – and because as a matter of fact, for the good of the child the context for reproduction is that of interpersonal union between the parents’. However, surely reproduction may not be desired here, even sub-consciously: it is much more plausible to say that reproduction is the purpose of desire. There are other difficulties: Pruss tells that, ‘Human reproduction involves ejaculation by the male into the female. In order to engage voluntarily in reproductive-type activity, it is plausible that one needs to intend the semen to reach the woman's reproductive system…’ (p.323). This sets the bar too high, for it would exclude all those who had no knowledge of semen or the specifics of conception (Pruss elsewhere appreciates this p.273) and thus no ability to intend this precise process – though they are certainly free of any intention to make their union non-reproductive or otherwise distort it. It is also a little surprising that Pruss favours semen collection via use of a perforated condom for married couples with fertility problems: he sees this as ‘entirely morally unproblematic’ (pp. 353–354), a view with which I would certainly take issue. For although the act resulting is minimally preserved as an act of a reproductive kind, it radically contradicts that total one-flesh self-giving union which Pruss's argument seems to demand (i.e. semen is deliberately withheld from the wife in the very act of union, albeit with a good further motive).

One becomes acutely aware in reading this book that sexual ethics is different from other areas of ethics, and radically so. The number of moral absolutes which proliferate in this area, in order to protect what some call ‘reproductive integrity’, is admittedly extraordinary. Pruss gives us excellent reasons, bound up with love and life and humane parenting, as to why this should be the case. I doubt I will read a better book on the philosophy of sex in my lifetime.