It's not true for Discovery in general. New phenomenon can be created and observed without any theory for why they occur, either before or after observation
- I need to observe here and not anywhere else
- I can reliably interpret my senses/the instrumentation is working correctly
- Objects of this type normally behave in X way, because of Y
- etc
Think of it a different way. You can come up with a theory without any observation whatsoever. Black holes, for example, were conjectured well before they were observed.
Thinking that something will either happen or not happen if I mix those two substances is not a theory.
OK I'll bite. What is it then?
Maybe an example would help clear things up. go into the lab with a 1 lb weight and a 2 lb weight and weigh them together.
Saying that the total weight could either equal 3 lb or any value other than 3 lb does not constitute a predictive theory for how the physics of summing Mass works.
It might be a theory that a scale display a value when I put things on top of it, but that is a different topic, and not what I'm testing.
This Theory doesn't tell me how the world works and if the expected value is 0 lb, 3 lb, or 1 million pounds.
I could go into the lab with no operating Theory or hypothesis on what the value of two masses should be when added together and collect data.
I can collect data with no expectation of correlation, and after measuring the combination of many weights, deduce that there is a relation between the combined weights and the total mass, and in fact it is a simple sum.
> Saying that the total weight could either equal 3 lb or any value other than 3 lb does not constitute a predictive theory for how the physics of summing Mass works.
The relevant theoretical background here is hidden in the "weigh them together" step: that there is such a thing as weight, it's described by a single real number, you can measure it in such and such a way, and so on.
You don't notice these considerations when it comes to weight and speed and size because they're hardwired into our brains by evolution. We're not so lucky when it comes to, for instance, the quark mixing angles - we can't even conceive of them without a background theory, let alone start measuring them.
If you want to count distant theories like "I exist" or "The world exists", then sure, every action starts with theory. But like I said, that is every different than a specific theory about what outcome an experiment has, and the underlying physics that make it so.
If you think nobody can discover anything without a theory for what could be discovered, you are flat out wrong.
Are you suggesting this was only possible because they had a "theory" that bread is not sweet?