You seem to be confusing subjective concepts with physical objects. In the typical bar magnet illustration of a magnetic fleld, you never sense the field itself, only its effect on iron filings. — Gnomon
On the one hand, fields are real and modeled mathematically:
"The fact that the electromagnetic field can possess momentum and energy makes it very real ... a particle makes a field, and a field acts on another particle, and the field has such familiar properties as energy content and momentum, just as particles can have....”
.....A “field” is any physical quantity which takes on different values at different points in space....
.....There have been various inventions to help the mind visualize the behavior of fields. The most correct is also the most abstract: we simply consider the fields as mathematical functions of position and time....”
(Feynman lectures, (CalTech, 1956), in Vol. II, Ch 1.5, 1963)
And on the other, fields are completely abstract and quantitatively incommensurable directly:
“....We now take it for granted that electric and magnetic fields are abstractions not reducible to mechanical models. To see that this is true, we need only look at the units in which the electric and magnetic fields are supposed to be measured. The conventional unit of electric field-strength is the square-root of a joule per cubic meter. A joule is a unit of energy and a meter is a unit of length, but a square-root of a joule is not a unit of anything tangible. There is no way we can imagine measuring directly the square-root of a joule. The unit of electric field-strength is a mathematical abstraction, chosen so that the square of a field-strength is equal to an energy-density that can be measured with real instruments. The unit of energy-density is a joule per cubic meter, and therefore we say that the unit of field-strength is the square-root of a joule per cubic meter. This does not mean that an electric field-strength can be measured with the square-root of a calorimeter. It means that an electric field-strength is an abstract quantity, incommensurable with any quantities that we can measure directly....”
(Dyson, EuCAP, 2007)
All that to say this: While it is true we never sense the field itself, I’m not sure that qualifies fields to be purely subjective concepts. I think perhaps we abstract the reality of fields from demonstrated characteristics of real physical objects. It follows necessarily that abstractions are immaterial, for the simply reason they are themselves irreducible to mechanical models, but nevertheless, that which is immaterial is not thereby merely a subjective concept.
Still, I suppose
all concepts originate in a subject, but calling them subjective concepts implies they have no use outside the subject that originates them, which is far from the case. The only purely subjective concepts are space and time, insofar as nothing causes time or space in the same way as physical objects cause fields. QFT refutes this, of course, but......one thing at a time, right?
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As an aside.....Kant didn’t know about fields, his natural science having to do with forces alone, without the conception of field associated to them.** So I wonder if he would have considered a field as a thing-in-itself, given what he actually did consider that way of things in general. I suspect not, for things-in-themselves are real objects of sensation to which our representations relate, but fields in and of themselves have no such reality of that phenomenal nature, in as much as their representations are actually representations of something else that is phenomenal, such that the conception of them becomes empirical. And, as we understand, representations of representations, are more commonly known as abstractions.
** See M. P. N. S., 1783, Pt II, Prop. 5, 6