Since the fall, human beings are said to be born with original or inherited sin. By whatever means we inherit this problem, we have it innately. We do not acquire it sometime in our lives. Paul tells us that Adam's sin affects the whole human species (Rom 5:12). Luther wrote, "all men are full of evil lust and inclinations from their mothers' wombs." Calvin wrote that original sin is "a hereditary depravity and corruption of our nature, diffused into all parts of the soul, which first makes us liable to God's wrath, then also brings forth in us those works which Scripture calls 'works of the flesh' (Gal 5:19)."
The doctrine of original sin (OS) is rightly regarded as a species of nativism. Nativism is the view that features of human beings are 'within us' and are not acquired. Those features might include capacities, dispositions, concepts, or beliefs.
According to a weak version, OS is the lack of a capacity--the inability to please God--and is only overcome by an act of God. A stronger version implies that OS is what causes sinful actions and not merely that all actions fail to please God. For example, Calvin says that OS "brings forth" sin. If so, then it must have some further meaning that accounts for a positive "inclination" to act in a sinful way. But what is it?
It is tempting to say that what's within is a desire to sin. But is it really plausible that an infant has such a desire? Desires appear to be acquired by children as they mature. If so, then desire is not a necessary condition for original sin.
One plausible answer is that OS leaves post-fall human beings with an innate disposition to perform sinful actions. For example, Wayne Grudem writes, "our nature includes a disposition to sin... Children do not have to be taught how to do wrong." But what exactly is a disposition? A disposition is commonly thought to be what we mean when we ascribe a property like 'fragile' to an object. Consider a glass. A glass is fragile. In other words, a glass has the disposition to shatter when struck.
What is odd about the disposition of a glass is that it never actually has to do anything to have the property of being fragile. What if it is never struck? It would never shatter but somehow it will always have the disposition to shatter when struck.
The simplest way to think about it is to consider a stimulus condition (the striking of the glass) and a manifestation of the disposition in question (shattering). Something like:
O is disposed to A when C iff O would A if C.
In the case of our glass, this is pretty clear: The glass is disposed to shatter when struck iff the glass would shatter if struck. Now, one might come up with all sorts of counter examples such as what might happen if the glass is wrapped in bubble wrap, but this just makes the analysis longer. One only need add 'not being wrapped in bubble wrap when struck' to make it work.
Consider a definition of OS using the simple analysis of dispositions:
A human being is disposed to sin when tempted iff the human being would sin if tempted.
The trouble comes down to what counts as the stimulus condition. Clearly, not every human being sins when tempted. Thus, temptation is not a sufficient stimulus condition. Whereas a clause added to the stimulus condition of a glass dispels the counter example in the bubble wrap case, it is not so clear that this would would for OS. One cannot add a clause that would ensure a tempted person will sin without referring to some other intrinsic quality of the human being. Furthermore, if temptation is sufficient for sin, then there is precious little reason we would have for holding a sinner responsible for his actions.
One might suggest this alternative:
A human being is disposed to sin when he desires to sin iff the human being would sin if he desires to sin.
It is still not clear that every human being sins when he desires to sin. Surely that is quite usual - the denial of our desires for a greater good and the avoidance of bad consequences. Furthermore, on this analysis, the stimulus condition is the feature of the human being we are calling a disposition. But this is what we are trying to analyze!
If there is no stimulus condition external to what we are trying to provide an analysis of then a dispositional analysis of OS is not possible.
Perhaps, then, we should turn to the idea of an ability or capacity. On the weak view of OS, we are said to lack the ability to please God. On the strong view we are said to have a positive ability to commit sin couched in terms of 'inclinations' or 'natures that produce sin'. But what is it to have an innate ability to commit evil? Plausibly, to have an ability is to have a power to act. Since only agents act, abilities are unique to agents. So, it could be said that strong OS is a kind of ability. This may in turn be analyzed in hypothetical terms:
For S to have the ability to sin is for it to be the case that S would perform a sinful action if S were to have certain relevant volitions.
On this view, a person has the property of being able to sin even though he or she may not actually sin. An infant who dies before performing a sinful action may nonetheless be said to have the ability to sin if he or she had wanted to. Furthermore, this analysis can account for the general condition of OS without ever being in a position to perform a sinful action. Consider a person who is never given the opportunity to sin. Could that person have OS? Though that person does not have a specific ability to sin, she still has the general ability to sin.
One might object that prior to the fall, human beings were untainted by sin, but they were able to commit sin (as demonstrated by their actual sin in the garden). But if being able to sin is what we mean by original sin, then we would have to say that pre-fall Adam was tainted by it. This is surely not the case given Paul's argument that sin entered the human species through Adam (Rom 5:12). Furthermore, how is it that we could ever be rid of original sin if it is an ability had by pre-fall Adam? Surely, if pre-fall Adam had it, then we will always have it!
In order to respond to the first objection, recall that the weak version of OS states that a human being has OS iff that human being cannot please God. What the strong version suggests is that there is an additional feature of human beings that is jointly sufficient for the doctrine. In other words, the doctrine of original sin is about both the lack of capacity to please God and the capacity to carry out wrongful actions. If so, then pre-fall Adam has the capacity to commit sin, but does not yet lack the ability to please God. Consequently, one might say that OS only applies to those who have the ability to commit sin and lack the ability to please God.
In regards to the objection about the future prospects of human beings, one might reply that though we presently have the ability to sin, we will lack this ability in our glorified bodies. At the resurrection of the dead, a glorified body is given to each person who belongs to the Lord. In that body, the person lacks the ability to sin and never lacks the ability to please God. In the meantime, believers are able to please God due to the imputed righteousness of Christ (who lacked the ability to sin, and never lacked the ability to please God).