This question is central to ethics. To be meaningful ethics requires that we have, at least to some extent, free will. That is the capacity to think, choose and act for ourselves. The scientific model of knowledge is based on the view that 'all events have a cause'.
Scientific research attempts to identify and describe causal relationships.
The scientific model is most evident within the physical sciences where scientists generally attempt to establish laws which can be used to predict physical phenomena. Laws of refraction, for example, are said to explain why a partly immersed stick appears bent when placed in water. The scientific model proposes a world in which, physical events at least, are predetermined. If humans are viewed as collections of cells, electrical impulses designed from a predetermined genetic blueprint then we too are the result of the interplay of a number of causal factors. In other words our actions, thoughts and choices are predetermined by some, as yet unknown, set of causal factors.