Atheism is a lack of belief in supernatural deities (i.e. God); the opposite of theism.
Atheists as a whole do not have a consensus on how to define atheism. A number of the issues involved in defining the term appear below.
Implicit and explicit atheismImplicit atheism is defined as a lack of belief in a god. Any person who has not been exposed to religious beliefs about the existence of god falls within the definition of implicit atheism. Atheists generally consider this definition inclusive of babies and little children who haven't yet formed opinions about the existence of supernatural deities due to a lack of exposure to them. Explicit atheism is defined as a conscious rejection of the existence of a god.
Strong and weak atheismStrong atheism is defined as a type of atheism in which a positive assertion claiming the nonexistence of a god is made. Strong atheists would make the statement, "God does not exist." Weak atheism is defined as a simple lack of belief in the existence of a god; it is a negative assertion. Unlike strong atheists, weak atheists would make a statement indicating a lack of belief; they would make the statement, "I do not believe in a god."Some weak atheists are strong atheists with respect to certain supernatural deities. Although weak atheists do not make a positive assertion that all types of gods do not exist, some choose to take a strong atheist's position and claim that a particular conception of a god (i.e. the Christian God) does not exist.
Epithetical uses of the termThe word "atheist" has also been used in an epithetical manner by various religious groups. The historical usage of the term as an epithet goes all the way back to accusations made against Socrates and others of his time. Early Christians claim to have been called atheists by pagans due to their lack of belief in the pagan gods of their time. 
Burden of proof
Most atheists argue that the burden of proof lies with the theist making the claim that a supernatural deity exists. In the atheist's view, it is a logical fallacy (argumentum ad ignorantium) to place the burden of proof upon the atheist.  Theists often try to shift the burden of proof to atheists by claiming that the atheist must prove that a supernatural deity does not exist. Atheists counter this argument by stating that it is a logical impossibility to prove nonexistence. To be able to prove nonexistence, the atheists claims, one would have to posses the ability to know all things perfectly (omniscience) and the ability to access all things simultaneously.  Since it is the theist making the claim of existence and since proving nonexistence is impossible, the atheist places the burden of proof upon the theist.
Common arguments against theism
Most modern atheists spend their time analyzing the Christian conception of God. In this light, atheists often put forward arguments that support atheism and oppose monotheistic religious beliefs. The Christian conception of God often presupposes several distinct characteristics: omnipotence (God is all-powerful), omniscience (God is all-knowing), omnibenevolence (God is all-loving), and omnipresence (God is all-present).
Atheists employ the use of logic to construct many of their arguments for the nonexistence of gods. An exhaustive list of arguments used to defend and support atheism is outside the scope of this article. However, below is an example of one logic-based argument commonly used by atheists in debates with theists.
Omniscience vs. free willAtheists may argue that free will and omniscience are incompatible. This argument is framed in the following manner:
- The Christian God is defined as a personal and omniscient being.
- Christians believe personal beings have free will.
- To have free will, one must have multiple options, all of which are avoidable.
- A state of uncertainty exists until choices are made and the potential to change choices exists.
- A being who is omniscient cannot be uncertain; the being knows its choices in advance.
- A being who knows its choices in advance cannot avoid its choices and lacks free will as a result.
- A being that lacks free will is not a personal being.
- A being that lacks free will is not a personal being, an omniscient and personal being cannot exist.
- Therefore, the Christian God does not exist. 
Atheists also often employ the scientific method to construct many of their arguments for the nonexistence of gods. In this approach, the atheist focuses on the lack of evidence for the existence of god or the errors in the evidence cited by some theists. Below is an example of an evidence-based argument commonly used by atheists in debates with theists.
Argument from nonbeliefAtheists may argue that, if a god did exist, there would probably be a number of nonbelievers in the world.
- If a God were to exist, there would not be as many nonbelievers in the world.
- There are many nonbelievers in this world.
- Therefore, God probably does not exist. 
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