Providing an evaluation of the argument we are asked to analyze (for the sake of brevity, let us call it argument “A”) is somewhat complicated by the fact that argument A is given incompletely, since some of its premises are clearly missing. Presumably, its general strategy consists in trying to show that if all events are determined, people's attempts at planning and affecting the future are unjustified, and consequently that not all events are determined. More formally, argument A's broad structure is as follows:
1) If all events are determined, people's planning in order to affect the future is unjustified.
2) But people's planning in order to affect the future is justified.
3) Hence, not all events are determined.
    One might object to the introduction of premise (2) by arguing that it is not stated in argument A. However, brief reflection shows that premise (2) is needed in order to infer conclusion (3) from premise (1).  Since as recast the argument is clearly valid (it's an instance of modus tollens), the only issue remaining concerns its soundness, namely, the truth of its premises. I shall argue that there are problems with both of them. In fact I shall provide some grounds for doubting the truth of premise (2); moreover, I shall show that premise (1) does not sufficiently distinguish between standard determinism and what I shall call ‘full fatalism”. As a result, one version of (1) 1 is true but another is false. Consequently I shall conclude that, in spite of its initial plausibility, while one version of argument involves full fatalism and is perhaps sound, the other involving standard determinism certainly is not.
    Premise  (2)  claims that what is certainly one of our most general and widespread form of activity, namely making plans and executing them in order to affect the future, is justified. One might try to argue that such behavior is so widespread that it must be justified.  But there is ground for doubting such a conclusion. Certainly we know of many examples of behavior in which whole generations of people engaged, and which were clearly unjustified. For example, consider the social practices, which lasted many centuries, involved in the apprehension, trial and burning of women thought to be witches. These practices were based in part on the erroneous belief that witches exist. Similarly our habit of planning for the future might be based on the false belief that our actions can affect future events. Generally speaking, that certain beliefs are widely held or certain practices widely followed does not entail that they are justified.  Moreover, the  continuing success of fortune tellers (just think e.g., of palm reading and horoscopes) seems to indicate that many people base their plans for the future on the belief that these charlatans can cast some light on how the future will be. Indeed, it looks very much as if, for some at least, planning for the future is inextricably and certainly at first sight irrationally connected with believing that future events are already fixed. Consequently, appealing to the common and allegedly justified habit of planning in order to conclude that the future is not determined loses some of its initial plausibility.  In light of the previous considerations, one can reasonably claim that the evidence for the truth of (2) is at best inconclusive. This is sufficient to cast some doubt on the soundness of the whole argument.
    While premise (2) is best characterized as doubtful, premise (1) has much more severe problems. In order to see why, it is necessary to analyze the view according to which events are determined. Broadly speaking there are two main versions of it namely, standard determinism and full fatalism.
    According to standard determinism, every event, including human choices, is determined by previous events.  More precisely, an event is seen as the result of initial conditions plus deterministic laws, i.e., laws which, given the same initial conditions, guarantee that the same outcome always occurs. For example consider a body at rest on a ledge 10 meters from the ground, and suppose it begins to fall. If one considers the laws governing the fall of bodies in such case, the velocity the body will have at the moment of impact is fully determined and will always be the same. ¹ It is important to remark that according to standard determinism, if the initial conditions or the governing laws were different the outcome might, although it need not, be different. For example, it the ledge were 12 meters from the ground, and the relevant laws remained the same the velocity at impact would be different. ²
    However, there is a particular version of the view that events are determined which differs significantly, and as far as argument A is concerned crucially, from standard determinism. The version in question is full fatalism, which holds that events, or at least some of them, are such that they could not be otherwise in the very strong sense that even if the initial conditions or the laws of nature were different in whatever way, the outcome would be the same. For example, if the body is fated to hit the ground with a certain velocity, it will hit the ground with that velocity no matter what the laws of falling bodies or its initial conditions are. So, while according to standard determinism the initial of conditions plus relevant laws have a bearing on the final outcome, according to full fatalism they do not.
    We are now in a position to see how a standard determinist would analyze (1). Presumably, the evidence for (1) is as follows:
4) If all events are determined, then what will happen tomorrow is already determined and nothing can change it.
5) Hence, what is determined to happen will happen.
6) Therefore, planning for the future is unjustified because the future is already determined.
A standard determinist would question the transition from (5) to (6), since for standard determinism what will happen tomorrow is partially dependent on what one does today. Believing the contrary is accepting full fatalism. A standard determinist would hold that what one does today is fully determined and contributes in turn to determine what will happen tomorrow. Whether I'll do well the course or not is already determined, but it certainly does depend on whether I study or not. Consequently, argument A is powerless  against standard determinism.  By contrast, argument A has some force against full fatalism, for certainly if our present efforts at affecting the future have no effect whatsoever, it is unjustified to act in order to affect it. And if one assumes that we do act justifiably when we plan for the future, fatalism must be false.  However, such an assumption might be denied by the full fatalist, and we have seen that the evidence for its truth is far from conclusive.
    The previous analysis shows that there are two distinct versions of argument A. The first is obtained by interpreting the idea that all events are determined in terms of standard determinism; the second by interpreting it in terms of full fatalism. I have shown that the first version is unsound and consequently does not prove that standard determinism is wrong. By contrast the second version is sound, and consequently refutes fatalism, if one assumes that our attempts at influencing the future are not unjustified.
 

Notes

1. Neglecting air resistance and variations in the value of g for the sake of simplicity, the laws at play are:

s=v₀t+1/2(gt²) and
v=v₀+gt
Simple computation shows that the velocity v at the moment of impact
will be *sg that is, assuming g=9.8m/s² ,14m/s.

2.  In fact, it would be 15.3 m/s. This does not entail that an outcome can be brought about only by the very same initial conditions and the very fame governing laws that made it occur. For example, in the case of the falling body, if the height of the ledge were 15 meters and the value of g were 6.5m/s², the final velocity would still be 14m/s. Indeed, by playing around with v= *sg, one can see that there is an infinity of initial conditions plus relevant laws which bring about the same outcome.