He argues that since putting someone into prison is the among the worst things society does to people, we need a good reason to do it. When it comes to using drugs, no such reason (has been, or) can be given. Alcohol, tobacco, coffee, and sugar are all drugs. Skiing is dangerous to one's health. Any reason given against drugs should not outlaw things we already think should be legal. He concludes that all drugs should be decriminalized.
A historic note: the prohibition of 1920's would be a good approximation of decriminalization the book argues for -- back then the consumers of alcohol were not criminals, only the distributors. Legalization (allowing for sale of drugs) requires a different argument than the book provides.
He argues (convincingly) that all drugs should be decriminalized. Putting someone in jail is the most extreme thing that our society does, and needs a serious justification. Every individual being put in jail deserves an answer, and no satisfactory answer can be given for putting someone in jail for a nonviolent drug use (or possession).
TL;DR version: anyone being put in jail deserves an explanation. None can be given when they are a non-violent drug user.
Surprisingly, the prohibition of alcohol in 1920's did not punish alcohol drinkers, just distributors. This model for drugs today would be a major improvement over the draconian treatment we have of the non-violent drug offenders.
We don't put people in jail because they drink (impairs the mind). We don't put people in jail because they ski (can kill people instantly). Putting someone in jail is the worst thing a state does to an individual, and an answer of "because we're not sure what will happen if we don't" isn't appropriate.
Read Legalize This: The Case for Decriminalizing Drugs (Practical Ethics Series) by Doug Husak
The gist: whenever someone is about to be put in jail for any reason, they ought to be given a reasonable answer to the question "Why am I being put in jail?" There simply is no satisfactory answer for current drug charges.
Bad answers to this question would require, out of consistency, criminalization of alcohol, tobacco, skiing (dangerous!), pizza (bad for your health!), etc.
I highly recommend the book Legalize This!: The Case for Decriminalizing Drugs (Practical Ethics Series) by Doug Husak
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