People are always telling you that "we have always done thus", and then you find that their "always" means a generation or two, or a century or two, at most a millennium or two. Cultural ways and habits are blips compared to the ways and habits of the body, of the race. There really is very little that human beings on our plane have always done, except find food and drink, sing, talk, procreate, nurture the children, and probably band together to some extent.
- Ursula K. Le Guin, "Seasons of the Ansarac", Changing Planes
Human cultures vary wildly and dicursively, so it is worth noting which things all known human societies have in common. Several generations ago, anthropologists noted that cultures' beliefs about a suite of concepts crudely describable as 'magic' had certain principles in common.
Humans seem to naturally generate a series of concepts known as "Sympathetic Magic", a host of theories and practices which have certain principles in common, two of which are of overriding importance. These principles can be expressed as follows: the Law of Contagion holds that two things which have interacted, or were once part of a single entity, retain their connection and can exert influence over each other; the Law of Similarity holds that things which are similar or treated the same establish a connection and can affect each other.
These principles are grossly, obviously, in contradiction with everyday experience. Thusly many cultures restrict the phenomena to which the laws supposedly apply to non-standard, special cases, most especially to individuals who it is asserted have unusual powers or ritual actions that are not commonly replicated in normal life. Examples range from African sorcerers could supposedly bring death to their enemies by stabbing their footprints, the Imperial City of ancient China which was designed to function as a stylized representation of the whole of the country and induce peace as long as the Emperor sat in his throne facing south, and all manner of witchcraft superstitions in which a discarded body part or tiny doll could be used to work magic at a distance.
Yet the laws themselves do not seem to be doubted, and the phenomena which they supposedly describe were historically (and often presently) widely believed despite a complete lack of actual evidence. There are even technical specialties which are not overtly "magical" where the laws were retained. An excellent example of this is herbalism, where a concept named "The Doctrine of Signatures" suggested that the form of a plant hinted or indicated what it was useful for.
Thus the Greeks thought orchids treated impotence and infertility because they vaguely resemble testicles, the Chinese believed ginseng was a potent panacea because its forked root looked somewhat like the human form, and medieval monks thought lungwort's similarity to ulcerated lung tissue meant it was effective against respiratory ailments. In many cases such beliefs persisted for centuries and across civilizations, despite there being absolutely no rational reason to view the beliefs as true.
Sympathetic magic is just a special case of a wider set of phenomena called magical thinking. It is important that you familiarize yourself with that collection of ideas before the next post.