I tend to hear two objections to the existence of immaterial things:
1.) Immaterial beings are unnecessary hypotheses
2.) Immaterial beings are unthinkable.
The first objection can’t be solved without some knowledge of the different methods of proof; and in light o this problem we most of all need to distinguish between a.) results that prove principles and b.) principles that prove results. The first kind of method begins with a hypothesis, the second kind doesn’t- and can’t. So far as one tries to start with the hypothesis of an immaterial being, he is almost certainly doomed, because then he must also lay down material beings as a contrary hypothesis, and insofar as we take material beings as hypotheses, we are agnostic about he extent and reach of their powers- we just have to wait and see what they can do. The one who looks a material beings in a hypothetical light can always imagine some day when matter will explain intentions or morality or universals or purpose, etc, and so far as he sees matter hypothetically there is no reason for him not to imagine this.