Dewey argues that the goal of science is not to figure out what things are, but rather to give an account of things where they have maximum convertibility with one another- i.e. the goal is to see all objects of thought as one thing or manifestations of one thing (we would say “where they are maximally reducible”). And so claims like “Heat is the average molecular motion” or “Jones is a swarm of electrical charges” are just like saying “That house is $140,000″ or “Plastics are a good investment”. The goal in statements like these is not to say what things are (which can, in fact, be a real goal) but rather set up a standard which allows for a maximum amount of homogeneity among objects. You can’t use dollars to define a house, but you can use them to bring the house into a domain where it is convertible with an indefinite ocean of other things, i.e. a place where everything is a larger or smaller manifestation of the same thing.
The claim goes too far if it is taken as meaning that science never gets to anything essential or intrinsic to things: electrical charges are essential to Jones in a way that dollars are not essential to a two-story Colonial. But even where science hits on the essential, it still only allows a part of it to filter through, and Dewey was right to recognize it as the part having maximum convertibility. We only look for aspects of phenomena that allow for maximal reduction. We were looking for electrons because they were all the same; we looked for the motion of molecules when we studied heat because it would allow us to unify heat and motion. We may have found them because they were there to find, but this was not the only reason we found them. Pure objectivity in the sense of simply saying what the thing is was never the only goal.
Notice that all of these distinctions are taking place within the context of the true. We might not be able to define a house by dollars, but it is nevertheless true that some house is $140,000. Just because one is not looking to define things or say what they are does not mean that they are not after truth. But we have to separate truth as the search for “what a thing is” from truth as the desire to achieve maximal analytical reduction. Science is a mixture of these two sorts of truth, and so far as we are scientists, we have no ability to distinguish them.