Immaterialists are usually quick to point out that immaterial entities cannot be interacted with or observed by material entities. Most Immaterialists will assert that science cannot detect, measure, or otherwise verify the existence of anything immaterial, because immaterial entities by their very nature are not composed of matter or material energy; they exist in a different dimension where material entities cannot go, or their qualities of existence make them unable to be interacted with by material entities. You cannot punch a soul. You cannot weigh it. You cannot observe the afterlife with a telescope, even if that telescope can clearly see every last inch of the entire universe. In short, Immaterialists claim that the nature of immaterial entities makes them a priori untestable and undetectable by any scientific or material means whatsoever.
I realize of course that some Immaterialists will not agree with the above paragraph. But the majority of Immaterialists (Christians, Muslims, Tarot-Card Reading Psychics, etc...) will agree with me that science cannot test or observe immaterial entities. For those Immaterialists who do believe that immaterial entities are subject to scientific testing and observation, I invite them to take the Million Dollar Challenge.
Now let's move on to the claim that most Immaterialists hold: That immaterial entities (souls, ghosts, etc...) cannot be tested or observed by material scientific means. I say to them, "That's fine, but the door swings both ways!" What I mean is that if material entities cannot test or observe immaterial entities, then immaterial entities cannot test or observe material entities either.
A typical Immaterialist will claim that a soul cannot be "touched" or "detected" by any material instrument, including your own material senses. Yet the Immaterialist will simultaneously claim (typically) that these immaterial entities can "touch" or "detect" your material self, as well as other material entities, including inanimate objects! The Immaterialist will usually claim that immaterial ghosts can speak to people, move objects, heal people, hurt people, and interact with the material world in all kinds of ways.
What is even more absurd is that Immaterialists will claim that all human beings (or even all living things) have an immaterial "soul" or "spirit" controlling their material body. How brilliant. A soul, that cannot be detected or interacted with by any material means, somehow is able to interact with a material body and make it breathe, think, masticate, defecate, copulate, and do all those messy material things that biological organisms do so well.
It is simply not logical to claim that there is a one-way street in regards to the interaction of immaterial and material entities. Consider the following symmetrical statements:
1) For you to be in my line of sight, I must also be in your line of sight.
2) To hold up this 10 pound object, I must exert 10 pounds of force.
3) If it is wrong for me to murder you, then it is wrong for you to murder me.
4) If I am your Son, then you are my Father.
Note that the above four statements are logically correct. Now consider the following asymmetrical statements:
1) For you to be in my line of sight, I need not be in your line of sight.
2) To hold up this 10 pound object, I need not exert any force.
3) It is wrong for me to murder you, but it is not wrong for you to murder me.
4) I am your Son, but you are not my Father.
Note that these four statements are not logically correct because they are asymmetrical.
Now consider these two statements, and consider which one is logical, and why:
1a) A material entity may not interact with an immaterial entity, and an immaterial entity may not interact with a material entity.
1b) A material entity may not interact with an immaterial entity, but an immaterial entity may interact with a material entity.
Statement 1a) is symmetrical, and logical. Statement 1b) is not.
Many Immaterialists will contend, and have contended, that immaterial entities do not operate along the same logical rules that material entities do. Fair enough, but it doesn't solve the problem. In fact, it makes the problem worse for them. Why? Because material entities do obey the law of symmetry; they are forced by their very material nature to obey it. The only way a material entity can interact is in accordance with the rule of symmetry. Consequently, for any other entity to interact with a material entity, it must also observe the rules that material entities observe. To be outside the laws of logic is to be excluded from operating within its realm, the material universe.
When dealing with two kinds of entities and the interactions between them, to place a restriction on one entity is to place a restriction on the interaction between the two, because the interaction between the two entities is dependent upon, and affected by, the properties of each entity. If immaterial entities cannot be detected, tested, or interacted with by material means, then material entities cannot be detected, tested, or interacted with by immaterial means.
When dealing with an Immaterialist who claims that immaterial entities cannot be detected or tested by material means, you can use this argument:
Premise 1: If immaterial entities cannot be detected, tested, or interacted with by material means, then material entities cannot be detected, tested, or interacted with by immaterial means.
Premise 2: Immaterial entities cannot be detected, tested, or interacted with by material means.
Conclusion 1: Material entities cannot be detected, tested, or interacted with by immaterial means.
Premise 3: Human beings are material entities.
Conclusion 2: Human beings cannot interact with any immaterial entities.
Conclusion 3: Human beings cannot contain or possess any immaterial entities, components, or properties.
But what about the Immaterialist who claims that immaterial entities can be detected and interacted with by material means? Simply ask him to support his implied assertion that immaterial entities even exist, and ask him to do it by material observation.
*UPDATE* Commenter Axel_621 helped flesh out my argument even further, and I wanted to post it in here because I think the observation that Axel_621 made is just so damn good.
I'd like to point out that if an immaterial entity exerts force on a material object, then the material object is also applying force to the immaterial entity by default. If this were not so, then no force could be applied to the material object by the immaterial entity.