In the essay "Sexual Perversion", Thomas Nagel attempts to form a general definition of what constitutes perversion. Nagel begins by setting up what he calls general conditions that the concept must meet without analysis in order to be evaluated under his view of perversion. They are 1. That any sexual perversion will be sexual desires or practices which are unnatural, 2. That fetishes of objects will be seen as perverse however unadorned intercourse will not and, 3. That perversion will be unnatural sexual inclinations as opposed to unnatural practices adopted not from inclination but for other reasons.

After this, Nagle forms a general definition of a normal adult sexual relationship by using an example of two people becoming aroused my simply noticing one another, deriving what he considers to be the basic framework of a full-fledged sexual relationship. Through this example Thomas Nagel demonstrates the steps of the development of a sexual relationship, all of which must be experienced (to a certain degree) for the relationship to be considered normal. In brief, Nagel says that a sexual relationship begins with the solitary arousal of one person, (the other person is unaware that someone has become aroused simple by noticing them) then the arousal of a person because they aroused another person, and finally the arousal must eventually become mutual and later physical contact or intercourse must occur. Thus, Nagel states that a normal sexual relationship must involve mutual perceptions. However Nagel points out that even if the above is the correct definition of adult sexual relations, it is not plausible to say that anything that does not fall exactly into this definition is perversion. He uses the example of sexual fantasies, to show this fault in his argument.

By indulging in the fantasy, one would not be recognizes one's real partner (thus there would not be mutual perceptions), meaning that this would be a defective sexual relationship, thus classifying it as a perversion. So because of this, Nagel goes on to add to his definition by saying that perversion constitutes not only the above but also the prevention of the development of normal sexuality (where there is definite potential for the development of normal sexuality) by distorting influences. Nagel says that the distinction between definite and indeterminate potential causes an area of un-clarity however this is where he ends his definition of perversion. Using all of the above, it is possible, says Nagel, to rule acts such as narcissism, voyeurism, exhibitionism, sadism and masochism. Nagel's reasoning for the ruling of the above as perversions is because narcissism, voyeurism as well as exhibitionism seem to be stuck in some primitive version of the first stage of sexual arousal since the experience focuses on the awareness of only one's own sexual embodiment. The later two are seen as perversions because they fall short of interpersonal reciprocity since sadism concentrates on the passive self-awareness of others (the reverse being true for a masochist) meaning that both exclude the awareness of one's self as an object of desire, what Nagel has determined to be the second step in a normal sexual relationship.

Nagel also points out (and seems to agree with the fact) that under his definition homosexuality cannot be ruled as perverse, since nowhere in his definition are mutual perceptions of the same sex ruled out. However, although Thomas Nagel believes that perversion should be evaluated to some degree, he is uncertain as to if perversion can be evaluated morally. He finishes his essay by stating that when it comes to determining the morality of perversion, it would require a greater understanding of both topics.