Determining if it is safe to cast a spell begins by looking at the type of spell. Is it a love spell or is it a curse? Is it a Wiccan spell or a Santeria spell? Although there are always exceptions, a good rule of thumb is that a curse is going to be less safe than a love spell. A spell that is intended to cause harm is going to be less safe than a spell intended to heal or protect somebody.
This leads to the second point - intention. The intention of yourself, as well as the intention of the person casting the spell, will influence the degree to which the spells manifests dangerous side effects. If your intentions are not entirely pure this can filter into spell castings, possibly even making the magic spells ebook. Even in cases where a person casts a curse out of anger, but doesn't truly want the result there may be some backlash. The most important aspect of intention is being in harmony with the result of the spell, rather than traditional ideas of right or wrong.
Intention on the part of the person casting the spell, if not the person having the spell done (in the case where a spell is done for one person by another), is also very important. If the person trying to cast a spell is not focused enough, distracted, or even harboring doubts and malice toward his or her patient a good spell could quickly turn sour. This is why it is important to find someone of the highest skill if you choose to have a spell done for you.
Whatever results from the spell will also determine if negative effects occur. For example, if you cast a curse and you harm somebody there might be a karmic debt you have to account for at some point. However, if you cast a curse and it ends up causing more good than harm (such as stopping a bad person from hurting others) then, despite being a curse, it is unlikely to have any long term harmful consequences.
The skill of the caster is also important in a more encompassing way. All of the former issues - impure intentions, type of the spell and even the end result of the spell can be accounted for with the right casting technique. This is a mixture of a person's innate ability, their devotion and time practicing their craft and the specific methods they have developed to cast spells that really work.
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