There is no absolute destiny: there is only Cause and Effect. Sometimes the process is simple like mixing blue and yellow will give green. However most of life's situations is not that simple. It is difficult if not impossible to discover a causal relationship. These are the situations for which one needs Tarot a means to see beyond the sensory perception.
The important thing to remember is "if you change the cause, you change the effect" i.e. if you change either blue or yellow (the cause), you change green (the effect).
The Tarot works, by helping the conscious mind focus, via meditation, on the clairvoyant powers of the unconscious mind. The underlying premise is 'solutions are inbuilt in the problem, one has only got to search for it.' Tarot is one of the tools, which help in this search .