You're assuming an idealization is a fictionalization, rather than a way of getting at an essential property (ie., a stable, real, causal feature) of a system.
Treating a cow as spherical is a means of selecting is real property of volume, as it is causally efficacious in say, a gravitational field -- whilst discarding is accidental-random variations in volume across all cows.
That we can treat cows as spherical, and obtain relevant dynamics should show that this early 20th C. instrumentalism is false. By idealization one selects the actual properties of objects for explanatory modelling -- one does not invent them or otherwise construct a merely instrumental fiction. Cows have volume, whose variation is accidental across cows, their volume expressed as a sphere selects better for their essential volume.
Very few, if any, theories of physics are predictive in almost any situation without this idealization -- because it is impossible to describe, eg., the volume of any actual cow. An actual cow has uneven density, shape, etc. and would require a significant amount of data to describe -- nearly all of which does not bare on the role its mass plays in a gravitational field.
What idealization does is create hypothetical scenarios which imagination all irrelevant causes are controlled, and all accidental properties are uniform (/ of a known distribution) -- so that the model can focus on Explaining the target Essential property in question.
These hypotheticals are not inventions, they are means of targeting what is being explained.
If you look at the predictive accuracy of scientific models, as applied in any actual scenario, they are fall apart -- almost nothing at all can be predicted, because all actual situations comprise innumerable accidental features which cannot be modelled.
Treating a cow as spherical is a means of selecting is real property of volume, as it is causally efficacious in say, a gravitational field -- whilst discarding is accidental-random variations in volume across all cows.
That we can treat cows as spherical, and obtain relevant dynamics should show that this early 20th C. instrumentalism is false. By idealization one selects the actual properties of objects for explanatory modelling -- one does not invent them or otherwise construct a merely instrumental fiction. Cows have volume, whose variation is accidental across cows, their volume expressed as a sphere selects better for their essential volume.
Very few, if any, theories of physics are predictive in almost any situation without this idealization -- because it is impossible to describe, eg., the volume of any actual cow. An actual cow has uneven density, shape, etc. and would require a significant amount of data to describe -- nearly all of which does not bare on the role its mass plays in a gravitational field.
What idealization does is create hypothetical scenarios which imagination all irrelevant causes are controlled, and all accidental properties are uniform (/ of a known distribution) -- so that the model can focus on Explaining the target Essential property in question.
These hypotheticals are not inventions, they are means of targeting what is being explained.
If you look at the predictive accuracy of scientific models, as applied in any actual scenario, they are fall apart -- almost nothing at all can be predicted, because all actual situations comprise innumerable accidental features which cannot be modelled.