This is understood readily if one considers that "harmless" means "unable to cause harm", whereas "harmful" means "causing damage or injury to somebody/something"; the first expresses something that is impossible, whereas the second expresses the idea of an ongoing effect from the moment that the interaction implied is established (absorption of the chemical or exposition to it, introduction of bacteria into an environment or body, etc.). A dangerous dog is not causing harm from the very moment it has been introduced into a human environment, and that without stopping; there is only a possibility that it might do so. Therefore the adjective "harmful" is not proper.
Addition suggested by users
No, the opposite of "harmless" cannot be "potentially harmful" in the case of dogs, which have to be called dangerous or something like that (when they are not harmless). Nor is it true in other instances: for instance, aspirin is usually considered to be a harmless drug, but it is potentially harmful (prolonged use is not really advised).
user Michael W.
This is true not only for dogs (and poisonous snakes) but for all sorts of things that share with dogs the characteristic of showing a potential for doing harm, but are harmless as long as in the environment where they are found no particular situation develops that will result in harm being done because of them; tanks (armored vehicles), bolts of lightning, x-ray machines, knives, and bleach, for instance, cannot be said to be harmful. There is the case of drugs, for which the two possibilities coexists, although not as both applying to the same sort of drug. The following example shows that a difference is being made between dangerous and harmful drugs.
Commonly used medications (aspirin, tranquilizers, pain killers, sleeping pills, etc.) are considered to be harmless in conditions of normal use, but they are all dangerous things: they are, especially, a danger to children, and situations can come about when harm to children will result from them (which is why it is advised to keep them out of children's reach); nevertheless, there is no justification in calling them harmful, and they are never labelled thus. There is however an established usage in speaking of harmful drugs: here again the same principle applies, drugs considered harmful cause damage as they are being used, no special situation is needed for that; most of the drugs sold illegally as mind altering drugs fall under this label.
There is another instance of thing to which "harmful" can and cannot be applied, and that is not relative to different sorts of the same thing but relative to different points of view: "bleach" (or detergents for that matter), which I mention above as not a substance that can be called harmful relative to the point of view of household use, when considered from the point of view of the contamination of streams can be talked about as harmful.
User linkhyrule5's comments and answer to them
- I would actually quibble with many of your examples in your reply on Michael W. While it's hard to say a tank is "harmful", I would certainly say a lightning bolt, or an x-ray machine in the context of overexposure, is harmful. –
Rather than making the distinction to be about the lack of a necessary context ("harmful things are unconditionally so), I would make the distinction to be about a lack of agency or choice. Bullets are harmful, lightning bolts are harmful, if you are a doctor ignoring the rules about standing in front of an x-ray machine the x-ray machine is certainly harmful. –
A dog or human is not harmful because they are living things that can choose whether or not to hurt someone, so they are dangerous and not harmful. A tank or gun is not itself alive, but it is a tool tightly bound to the user (c.f. " doesn't kill people, people kill people") and so falls under the same category.
Your objection in the case of the x-ray machine (I mean by that medical equipment) makes absraction of the fact (understated in my post) that they are used properly: normal exposure of a patient to X-rays results in no harm, and the danger is materialized only on the condition of an important accumulation of exposures; likewise those machines are harmless to the operators since normal use consists for the operator in remaining sufficiently far from the machine when it is operating (so as to avoid the accumulation of exposures patient after patient as if near the patient the operator is also affected by the rays).
If you now add the precision "in the context of overexposure" you come up with another example of those "harmless/harmful" cases that depend on context (aspirin, bleach), and I can't but agree with you (since this is a point I make in my post); in a context of abusive utilisation harm does result.
As concerns lightning, exposure is not truly a reality as far as I understand this phenomenon, and I prefer the idea of "a hit by lightning"; anyway, here again, the activity is that of the usual occurrence in nature (there seems no other possible) and nobody has ever complained about the occasional tree that is destroyed by lightning—at least since Benjamin Franklin—, nor about the (much rarer) forest fire that can result from such an incident; there does not exist in my opinion a tangible notion of harm as harm being done to nature; so lightning is not harmful but dangerous for animal and human life.
I do not make the distinction between dangerous and harmful on the basis of context; I make this distinction on the basis of the normal activity of the agent. The activity of a dog cannot be causing harm at every instant of its existence, and this can occur only on certain occasions that are created in its environment; on the contrary, a chemical agent, as it contaminates the environment, by its activity, will cause harm to the environment as long as it remains active. It is a distinction as to what agent is involved if you are willing to ignore the agent's activity, but I find this distinction not fundamental, and that the type activity from which harm results is the key in deciding whether one or the other of the two adjectives applies to a particular agent. If you are talking of agency and choice as relates to ultimate human and/or animal responsability you exceed the bounds of the question, which is to evaluate the nature of the agent per se as constituting a danger or as doing harm. Human or animal responsabilities involved do not impinge on the decision as what this nature is.
I do not think you can say that bullets are harmful things. Even in the case of their being used to defeat an ennemy (let's assume, rightfully), while harm is being done to the ennemy (from any point of view), it seems unthinkable to call them harmful when in fact they contribute to avoiding further harm that could be incurred by those that have to defend themselves. For example, I couldn't imagine a poet narrating the feats of the fighters that liberated his country in such terms as "While our harmful bullets mowed down the ennemy…"; this is akin to the notion of doing wrong to somebody, and it just doesn't apply here. However, I would agree with such truths as "cartridges are dangerous things, and children should be prevented from playing with them".
In deciding that an x-ray machine is certainly harmful you have to appose the condition "if …", and this is tantamount to saying that an x-ray machine operated in disregard of safety rules is harmful, a statement with which I agree, but this is not "an x-ray machine".
Your preference for a distinction founded on the power of decision becomes finally unmistakable ("A dog or human … dangerous and not harmful."). In my opinion, this is not the correct approach. I will now provide an example that will show that man can be harmful independently of his decisions, although "man" is taken as the species. The presence of Western civilized man has been shown to be the cause of mass destruction of the natives of the New World because of the new diseases they brought with them (A consequence of European colonization); it is also known that today the presence of the same sort of people is restricted in some primitive tribes because of similatr reasons. It can be said then that (the presence of) civilized man among tribes of primitive men and such like ethnicities is harmful to the existence of these beings from the mere fact that civilized man is a carrier of numerous diseases against which primitive man has no natural immunity. According to your point of view, the colonizers, who were unknowing as to this state of affairs, because they could choose to do harm or not, were not harmful; this is not true, their presence was harmful: even if the process of contamination is not a thing of every instant, as is for instance the inhalation of a toxic gas, it still has a continuous effect endowed of a certain regularity; this is an aspect of their activity, and it does not matter whether it is conscious or not as far as its effect is concerned. In the modern case of a group of westerners (also ignorant of these medical facts) spending some time among a tribe of primitive people, for example in view of establishing some commercial relations, I agree that within some limits one might describe the situation rather as very dangerous, or say that these men constitute a danger to the health of the primitive population, but studies show that in the long run (which might not amount to such a long time after all), harm will be done. Those people's presence is bound to be harmful. This contradiction shows that harmfulness is not primarily a matter of decision. Anyway, plainly, excessive concentrations of chlorine in streams kill fish and it is not a matter of the power of decision of anyone to conclude that chlorine is harmful to fish.
If a dog can be considered to cause harm by choice in conditions that do not warrant a defensive behaviour, then it's a dangerous dog; opposing this idea of choice to the idea of context is not right; for one, context becomes relevant in my explanation only when considering that there are things that can be harmless and harmful according to two or more possible contexts, or harmless and dangerous in a similar way. This is not a criterion concerning the choice between "dangerous" and "harmful". Then what about such an obvious statement as "In Africa the elephant can at times be harmful to both crops and population."?
In fact we can see that context is relevant in other situations involving a single dog. There is no case for a dog which in normal conditions will be harmless and dangerous (contradiction), and we have to bring the discussion onto another level and speak of certain trained dogs that will do no harm to anyone unless they are told to do so by a human being; then clearly it is a matter of context that allows to speak of a dog both harmless and dangerous. This last remark was merely intended to show a little more of the complications that lie beneath the surface, in particular as context can open other perspectives.