Rachel Lee Pierce said:
Can you elaborate or back that up?
A particle's state is essentially a vector. And, any vector can be expressed in any basis. You might say that something like the vector ##(1, 0, 0)## is special because it can be expressed as a single vector in the normal basis. Whereas the vector ##(1, 1, 0)## is the linear combination (superposition) of two basis vectors: ##(1, 1, 0) = (1, 0, 0) + (0, 1, 0)##.
But, if we look at these vectors in a different basis, then either or both are a linear combination of the basis vectors.
For example, if we take the simple vertical force of gravity, we can decompose it into the linear combination (superposition) of a force normal to an angled surface and a force tangential to the surface.
So, is gravity a single vertical vector or is it the linear combination of two other vectors? The answer is that at all times gravity has an infinite number of decompositions as a linear combination of any number of forces.
The same is true for the state of a particle. It is always a single state
and a linear combination (superposition) of the states of any basis. Sometimes a state coincides with an important basis state (like an energy eigenstate) and that makes it physically special. But, it's still a superposition in any other basis other than that of the energy eigenstates.