You can only believe in purgatory if you hold a substance dualist view of human beings.
Purgatory is a place that exists according Roman Catholic Theology, and a number of people who are not Roman Catholic believe in it, too. In Catholic theology, it is a place where you go after death if you are not yet ready for heaven, so that you can receive punishments for the venial sins (the less serious sins, as opposed to mortal sins) that have not yet been dealt with in this life. As Thomas Aquinas put it,
[I]f the debt of punishment is not paid in full after the stain of sin has been washed away by contrition, nor again are venial sins always removed when mortal sins are remitted, and if justice demands that sin be set in order by due punishment, it follows that one who after contrition for his fault and after being absolved, dies before making due satisfaction, is punished after this life. Wherefore those who deny Purgatory speak against the justice of God: for which reason such a statement is erroneous and contrary to faith.
Outside of this historical Catholic understanding of purgatory, others have suggested, not that people need to be punished, but rather that they simply need to be fully sanctified (made holy) before reaching their final state in heaven. Jerry Walls defends this view in his book Purgatory: The Logic of Total Transformation. In public conversations, Dr Walls has remarked that while no doubt the sinful human desire is to have total transformation all at once, the reality is that sanctification is a process that takes time, hence purgatory.
I do not believe in purgatory, but I will not here argue that purgatory does not exist. Instead, I will just make one observation: To believe in purgatory presupposes mind-body substance dualism.
I am taking it as given that Christians who believe in purgatory, as well as those who do not, regard the teaching of the New Testament as authoritative. When Christ returns and the dead are raised, according to St Paul in 1 Corinthians 15, both those who were dead and the saints who are still alive will be transformed immediately.
So is it with the resurrection of the dead. What is sown is perishable; what is raised is imperishable. It is sown in dishonour; it is raised in glory. It is sown in weakness; it is raised in power. It is sown a natural body; it is raised a spiritual body…
I tell you this, brothers: flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable. Behold! I tell you a mystery. We shall not all sleep, but we shall all be changed, in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised imperishable, and we shall be changed. For this perishable body must put on the imperishable, and this mortal body must put on immortality.
The transformation to glory, honour, power, imperishability etc that is envisioned here is said to happen “in a moment, in the twinkling of an eye,” which I take to mean very quickly, more or less straight away. So a gradual process of transformation to glory seems out of the picture after the resurrection of the dead.
But this does not trouble traditional Catholic theology, since Purgatory is believed to be a place that exists now, and right now the souls of those who will eventually see glory are being purified. This is why Catholics are urged to pray for those in purgatory, and why masses have been said for them, so that their time in purgatory might be made shorter. Similarly, neo-purgatory (as I am wont to call Jerry Walls’ view) can accommodate a period to transformation as one approaches glory prior to the resurrection, when transformation to glory and power etc will occur “in a moment,” provided it locates such transformation in an intermediate state between death and resurrection, as does Roman Catholic theology.
What any such view presupposes, obviously, is that I can exist, I can think, I can be held responsible, I can be punished, I can undergo moral and spiritual improvement, etc, while my body is no more. That won’t come as a shock to people who believe in purgatory, because this is exactly what they believe about themselves. This view is mind-body substance dualism (since the “I,” the mind or soul, is a thing that can exist without any physical substance).
But anyone making a case for purgatory (and anyone considering that case) should consider this fact. In order to believe in purgatory without tension with your other beliefs, you must hold (or adopt, if you do not yet hold) mind-body substance dualism. To the extent that you have a reason to reject that view of human persons, you have a reason to not believe in purgatory.
Or so it currently seems to me.