To get a patent for your invention, it must be patentable. Fairly obvious, but what determines whether an invention is patentable or not? The main requirements of patentability are that the invention must be new and it must be inventive over everything else that is publicly known.
Showing that the invention is new is straightforward and relatively easy. A single feature of the invention that is not already known, even if that feature is trivial, is sufficient to provide the required novelty. Imagine that all bicycles in the world are, and have always been, red. A blue bicycle would be new. The novelty can even relate to a new combination of features, even if all the individual features are already known.
But the invention must also be inventive. This means that the new feature (or new combination of features) must not be obvious. A blue bicycle is not inventive. However, if the blue colour somehow made the bicycle more aerodynamic and go faster (it doesn’t), it could be deemed inventive. This is because the new feature provides a non-obvious technical advantage.
I said “could be deemed inventive” because it could depend on how the inventor knew that blue bikes would go faster. If the inventor conceived a new theory of physics which predicted this, or even just had a hunch which paid off, then the invention would be deemed inventive. But if they read about it in ‘Bicycles’ magazine then it would not be.
Inventiveness is viewed through the eyes of an entirely fictitious character known as ‘the skilled person’. Skilled persons cannot invent. They are the least imaginative persons in the world. But they do have the best memory in the world and somehow are able to know absolutely everything that is publicly available in their technical field. They even know intimately all of the contents of all relevant earlier patents.
The technical field of the skilled person depends on the nature of the invention. If the invention is a bicycle, then the skilled person is a designer of bicycles and has all (but not more than) the usual skills associated with such a job. They will also have a degree of technical knowledge of related fields such as manufacturing processes used to produce bicycles, the physics of motion, and of vehicles in general.
The question that is asked is – would the skilled person, starting from technology that is already known, have been capable and motivated to modify the known technology in such a way that they arrive at the invention? If such modification would have been simple or routine, or obvious because there is some guidance from research in the relevant technical field, then the invention is not inventive.
This all might sound vague and uncertain, but in reality it is not (at least most of the time!). Patent attorneys and patent examiners are trained and experienced in this and they all apply the same rules in much the same way. The result is that, if we know what is already publicly known, we can usually be fairly certain whether or not an invention is likely to be deemed patentable.