Plato was mistaken, or at least his concept of forms (ideas) was.
The mistake was in the relationship between physical reality and innate ideas. The concept is understood to be that the thing of the reality is derived from the innate idea (forms). The concept is that forms are the true reality, and the physical world is but its shadow, or reflection of it.
Modern science has shown where ideas are created — in the brain as the mind. The more experience we have the better our ideas of the things of physical reality. Each particular thing categorised becomes a general idea of the thing. It is within the name-meaning (sign) that which our knowledge is based upon.