Karl Popper once said that the notion of truth is counter-productive in science because it brings the/a scientific story to an end. Instead Popper talked about verisimilitude. However, many would argue that the notion of truth is actually contained in the notion of verisimilitude. In fact Ernan McMullin (in his 'A Case for Scientific Realism') makes that point point when he writes that the
“term 'approximate truth'.... is risky because it immediately invites questions such as: how approximate, and how is the degree of approximation to be measured?”
Yes, truth is hiding in the background here. Not only that: in order to know that a theory is approximately true, wouldn't one also need to know what would make it completely true and what makes it partly false? And in order to know that, wouldn't one also need to know what the complete truth of that theory is? Therefore, on this reading, there would be no approximation or verisimilitude about it. In any case, Ernan McMullin puts the same – or a similar – point. He writes:
“I do not think that acceptance of a scientific theory involves the belief that it is true. Science aims at fruitful metaphor and at ever more detailed structure. To suppose that a theory is literally true would imply, among other things, that no further anomaly could, in principle, arise from any quarter in regard to it.”
Strictly speaking, only statements and propositions can be true or false. Nonetheless, if a theory is seen as a simple collection of true statements, then it can be seen as true if all the statements it contains are also true.
Yet scientific theories don't work like that.
Even though theories do contain statements, not all those statements can be seen as being either true or false. Some involve predictions, probabilities, conjectures and whatnot. Only few of the statements or expressions which make up a theory have an entirely truth-conditional content.
If we bear in mind the McMullin quote above, it's also clear that a “fruitful metaphors” can't be true either.
What McMullin also seems to be arguing is that a “worldly structure” shows itself only slowly – over time. Each successive theory about the (same?) structure comes closer to the truth. However, since this is ongoing process concerning theories about structure x, then no single theory of x can ever be said to be conclusively true.
McMullin gives an example of this. He says that
“[s]cientists in general accept the quantum theory of radiation. Do they believe it to be true? Scientists are very uncomfortable at this use of the word 'true' because it suggests that the theory is definitive in its formulation”.
In other words, scientists don't need to classify their theories as true. Indeed it can even be said that scientists don't need truth at all. Instead a scientist can “accept an explanation as the best available”. Moreover, “one accepts a theory as a good basis for further research”.