A reaction in the colloquial sense is just an action. A reaction in the sense that Kingace invoked draws from the wording of Newton's 3rd law which is the definition I noted. I mentioned the true randomness of the coin flip as a side note. Under classical understanding, the flip is determined. However, quantum mechanics does claim that particle behavior is random. When you get to very large bodies moving slowly like coins, the probability of things behaving not classically is not zero, but extremely small. Delving deeply into quantum physics wasn't particularly necessary, but I put it out there as something to consider. Even then, the behavior of the coin is more or less fixed, in that the probability distribution is quite defined. There have never been unicorns, nor will there ever be unicorns. Yet I can imagine a unicorn. If you can't imagine unicorns or perfectly random coins, then I don't know what to say. The toss of a perfectly random coin is truly random if there is a 50% chance of it landing heads, and a 50% chance of it landing tails. You can also say that it can randomly turn into a unicorn. But the probability of that in our distribution is zero, so we don't really talk about that case. Saying that coin tosses are actually determined doesn't mean we can't understand randomness. Randomness only makes sense with respect to a probability distribution. Why should a person assume a distribution where a coin has a non-zero chance of turning into a unicorn over a distribution where a coin has a zero chance just because he is an atheist? Both distributions can be "truly random." Just because one distribution has a non-zero chance of X event happening doesn't make it more truly random. The point of my post, anyways, was to just set things straight about science that KingAce is wrongly using to advance his argument. If you don't understand the premises you are arguing on, it's probable that you don't really understand whether your argument is a good one in the first place.