Arguments for the existence of God customarily are framed in a priori or a posteriori ways. Theoreticians base a priori arguments on concepts, general principles, and rigorous analysis of terms. All such contemplative activity involves the use of one's mental processes. Anselm of Canterbury famously began with the concept of a perfect being, then reasoned from that concept to the actual existence of God. Whether readers believe he was successful in this endeavor, the point is that his approach exemplifies what it means for an argument to be framed by using a priori concepts.
On the other hand, some thinkers choose a posteriori argumentation to demonstrate God's existence. A posteriori (a Latin expression like a priori is) could be defined as starting from sensory experience, that is, appealing to features of the cosmos or reasoning from effects to their ultimate cause. The notable five ways argument of Thomas Aquinas exemplifies a posteriori reasoning. This study will provide a basic introduction to those five ways: I wrote these lecture notes for an undergraduate audience.
Aquinas contends that the existence of God is self-evident. However, it is possible to understand something being self-evident in two ways: a thing may be self-evident in itself (per se nota) or self-evident for us (). To illustrate, the Pythagorean theorem is analytic (true by definition), but does that mean its analyticity is known by everyone? Most would probably say that is not the case. So it is possible for a term to be self-evident in itself, known per se nota, without the term being self-evident to us. Aquinas reasons that God's existence is comparable to the Pythagorean theorem or statements of logic. As another example, students often think the statement, "All bachelors are unmarried men" is possibly false--not true by definition. But once we define "bachelor" as unmarried men, then it only follows logically or analytically that the statement is self-evidently true in se, even if it is not self-evident for us. Aquinas proposes that God's existence is analogous to these examples. Since God's existence is not self-evident to all, it seems that the preferred approach to proving there is a God involves a posteriori reasoning. Hence, the five ways of Aquinas.
The magnum opus of Aquinas is the Summa Theologiae (also known as the Summa Theologica). I will use ST to abbreviate the work: it can be translated "Summation of Theology." One distinctive feature of ST is the famed scholastic method--a method that entails setting forth questions, anticipating objections, posing counterpoints to the objections, quoting philosophical and ecclesiastical authorities and Scripture, then giving a response before addressing each anticipated objection. Aquinas follows the method for 613 questions, which apparently are meant to resemble the supposed 613 mandata of Jewish Law.
Having discussed basic features of ST, I will now discuss the five ways argument, an argument that is cosmological rather than ontological.
1) Aquinas' first way is the argument from motion (motus), something that we all experience. One can define motion in this context as "any change whatsoever" or "a reduction of potentiality to actuality" (using Aristotelian categories). The basic point is that everything in the natural world is moved by something else other than itself: no potentiality becomes actuality without an agent of change. For example, Aquinas reasons, a log is potentially hot, but in order for the potentially hot log to become actually hot, an agent of change that is actually hot must reduce the potentially hot log to being actually hot: in other words, fire is actually hot. Therefore, fire is able to make what is potentially hot, actually hot. The same principle would apply to a potentially wet towel. It could only become actually hot by water as an agent of change. But how do these examples apply to demonstrating God's existence?
a) Invoking the categories of motion (change), potentiality and actuality, Aquinas reckons that all motion in the world needs an explanation. My foot presses the gas pedal, which revs the engine, which moves the car. But what makes my foot move? And what causes the agent of change that moves my foot, and so on? Now either one accepts an infinite regress of movers or the reasonable conclusion is that God is the first unmoved mover. One thing that makes the argument probative is that if there is no first mover, then there is no intermediate mover. However, since there are intermediate movers, then a first mover must exist. Quote scholar. Kenny and inertia objection.
2) The second of the five ways is the argument from efficient causes. At this point, Aquinas again shows his conceptual dependence in large part on Aristotle. The Philosopher discusses four types of causes in his works Physics and Metaphysics. These four are the material, efficient, formal, and final causes. By efficient cause (causa efficiens), Aristotle means that cause which brings something into being or sets a thing/event into motion. To use one of Aristotle's examples, a father efficiently causes his child to exist. We would now say that parents jointly constitute efficient causes, ceteris paribus. The important consideration for Aquinas in this case is that X cannot be an efficient cause of itself: i.e., a child cannot be his/her own efficient cause.
b) Imagine someone playing billiards. Neural activity causes the player's arm to move; the player's arm causes the stick to move; the stick efficiently causes the ball to move; the ball travels to the side pocket (an effect produced by the first efficient cause). I am simplifying all of the events that occur with this seeming rudimentary occurrence, but the point is that in the case of playing billiards, we have a first cause (plausibly), then intermediary causes, and there's a final effect. Now Aquinas reasons that either we posit an infinite regress of efficient causes or we reason that God is the first efficient cause. Which option should we take?
c) Aquinas contends that if there is no first efficient cause, then no intermediary or ultimate causes exist, nor any ultimate effects:
Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false.
So positing God as the first efficient cause seems better than positing an infinite regress of efficient causes. If there is no first efficient cause (e.g., if neural activity never causes my arm to move), then no other causes are able to exist (e.g., no eight ball in the corner pocket). The acceptance of a first efficient cause by Aquinas sets him apart from Aristotle, who only views God as a final cause (telos).
3)The third way is tougher to understand than the first two. It "is taken from possibility and necessity." Two basic types of existence are possible being and necessary being:
It is commonly accepted that there are two sorts of existent entities: those that exist but could have failed to exist, and those that could not have failed to exist. Entities of the first sort are contingent beings; entities of the second sort are necessary beings (Matthew Davidson, Stanford Encyclopedia of Philosophy).
In place of "contingent," we can use "possible," but the point remains. Certain beings (entities) possibly exist or they are possible objects, whereas other beings are necessary, which means "could not have failed to exist."
a) Aquinas reckons that it is impossible that all entities only exist in a possible sense, otherwise, at one time nothing existed and nothing would now exist if all entities were possible, but not necessary. How does Aquinas reach this wide ranging conclusion?
b) One assumption is that a possible (i.e., contingent) entity at one time did not exist. For example, my existence is contingent or possible: I exist, but might not have existed, if circumstances had differed. My future existence is also contingent,that is, not necessary since it may terminate at any time. Another aspect to my possible existence is that there was a time when I did not exist. However, what is true in my case seems true of other beings like me whether they are trees, dogs, cats, bears or other humans.
c) If possible entities once did not exist, whence their course of life? How was their possible existence actualized? For a possible being only comes into existence in virtue of an actual being--an entity that already has being. Hence, there must be a necessary being whose existence is not simply possible, but actual.
d) Appealing to the infinite regress idea again, one can reason that either an infinite regress of necessary beings exist "whose necessity has a cause" (Aquinas). Alternatively, one could reason that there must be an entity such that the entity is necessary per se (through itself), and such an entity's reason for existence is not ab alio (from/by another). This per se necessary existent is known as God.
For a detailed formalization of the third way and other ways, see https://www3.nd.edu/~jspeaks/courses/2009-10/10100/LECTURES/4-third-way.pdf
A formal version of the third way might look this way:
1. There must be a necessary being, God, which can’t not exist and which keeps everything else in existence.
2. If something is contingent, then it can fail to exist, and there must be a time at which it doesn’t exist.
3. If everything were contingent, then there must have been a time at which nothing existed.
4. Everything has to be caused to exist by something existing before it.
5. If there was a time at which nothing existed, then nothing would be in existence now.
6. Things are in existence now.
7. It must not be the case that everything is contingent.
The last link posted above evidently accuses Aquinas of what logicians call "the quantifier shift fallacy." To avoid such problems, Robert Maydole suggested that the third way of the cosmological argument be modalized (i.e., frame the argument using modal logic).
4) The fourth way is the argument "from the gradation to be found in things." Aquinas insists that entities are more or less good, true or noble. Objects are hot, hotter and hottest. In order for the concepts ""more and less" to be intelligible, there must be an entity, which constitutes a maximum for other entities, so that gradations and comparisons between entities make sense. In other words, speaking of one fever as hotter than another fever only is intelligible if we have some maximum standard for hot objects. The same principle applies to entities that are more or less good, true or noble. Beauty might also constitute another example that illustrates the fourth way. We often make judgments that one painting is less/more beautiful than another painting. What allows us to make these kinds of aesthetic judgments? Could there be an overarching type of beauty that functions as a maximum whereby all objects can be judged against it?
Source of Picture: Wikipedia Commons