Mill's central theme in the essay is what he calls the ‘very simple principle’ of liberty. According to the principle of liberty, ‘damage, or probability of damage, to the interests of others, can alone justify the interference of society’. Mill offers greater protection still to expressions of opinion. Interference with these, contrasted with actions in general, is legitimate only when ‘the circumstances in which they are expressed are such as to constitute their expression a positive instigation to some mischievous act’. This is a stronger criterion than the one provided by the main liberty principle for actions in general.
There are important questions about how these principles should be interpreted. While the ‘very simple principle’ is indeed simple to the extent that it is not complicated, its import is elusive. Conscious of this, Mill restates it in a variety of ways through the essay and devotes the last chapter of the essay to a series of applications intended to clarify its ‘meaning and limits’.
Overall, Mill's explanation of his principles is clear enough, but translating them into detailed policy then raises new questions. Mill’s principles plainly have some controversial implications. For example, they rule out appealing to the addictive and self-injurious nature of drug use as an argument for (as against drug dealing) illegalising it. Likewise, they permit freely agreed assisted suicide, unless it could be shown that such suicide would be harmful in some way to people other than those freely involved in the collaborative act. Further, Mill would no doubt have opposed legislation against incitement to racial and religious hatred as unacceptably diffuse, since he held that the law should focus specifically on positive instigation to wrongful acts.
On the other hand, in some cases Mill is less permissive that we are. He believed it to be, for example, a ‘moral crime’ to bring into existence a child if one is unable ‘not only to provide food for...