Yes, because in the ordered pairs (x,y) all the "y" (dependent variable) values are unique and so are all the "x" (independent variable) values. This means that, according to the given points, each x value maps to one and only one y value and each y value is mapped by one and only one x value. A function can only map an input (x) to one unique output (y). The same x value cannot map to two different y values, although two different x values can map to the same y value. So, even though we may not be able to guess the function, the definition of a valid function applies here.