PRINCIPLE OF SUFFICIENT REASON

Principle of Sufficient Reason (PSR): For every thing that exists, there is a sufficient reason/explanation/ground for its existence or occurrence.

The reasons that fulfill the PSR are divided into 3 types as listed below. By laws of logic, the list is exhaustive.

• Type 1: External necessary reason: The existence of a thing is explained by causal necessity. E.g. a rock exists because molecules are bonded together by laws of physics and chemistry.
• Type 2: External contingent reason: The existence of a thing is explained by a free choice or by design, and for a specific purpose. E.g. a paper-cutter exists because man designed it, for the purpose of cutting paper.
• Type 3: Internal reason: This applies to all tautologies. The existence of a thing is explained inherently or by its own objective definition. E.g. the formula "2+2=4" exists necessarily because II and II are inherently found in IIII. Or, should there be a certain being who has existence inherently, then the statement "this being-that-inherently-exists exists" is a tautology and is therefore necessarily true.

A full description and defense of the PSR can be found in this post.


APPLICATION OF THE PSR IN HISTORY:

In the pre-modern period, before the 1600's:
People did not know about atoms and laws of nature (e.g. laws of physics, biology, chemistry, and evolution), and thus did not explain the existence of things in the world by causal necessity (type 1). They also understood that they did not inherently exist (type 3) because things would come and go out of existence by transforming into other things. E.g. water transforms into vapour and vice versa. Therefore, they concluded that all things in the world were designed (type 2).

With that, the existence and behaviour of things were described using Aristotle's four causes:
1. Efficient cause: what we now simply call "cause" as in cause and effect.
2. Material cause: what it is made of.
3. Formal cause: the identity or design of a thing.
4. Final cause: the purpose or function of designed things.

For example, the existence of water is caused by vapour or clouds (efficient cause), is made of a certain amount of matter because it has a certain mass (material cause), is a substance designed to be fluid and fall to the ground (formal cause), and must have a purpose because all designed things have a purpose even if we don't always know what that purpose is (final cause).


Since the modern period, starting in the 1600's:
Aided by the rise of scientific instruments, we now know about atoms and laws of nature. Thus, we explain the existence of most things in the world by causal necessity (type 1). With that, we no longer need four causes to describe the existence and behaviour of things but only two:
1. Efficient cause: what caused it.
2. Material cause: what it is made of.

For example, both water and vapour are composed of the same molecules H2O and energy (material cause), and their existence and behaviour are explained by the laws of physics and chemistry (efficient cause).


THINGS MADE BY INTELLIGENT DESIGN:

Despite our understanding of modern physics, are there still things in the world that fit the old Aristotelian model of having a formal cause and final cause, i.e. of being designed with a purpose? I can think of three groups of things:

1. The fundamental laws of nature: Most things in the world are explained by causal necessity because they obey the laws of nature. But what about the fundamental laws of nature themselves? They cannot obey more fundamental laws by definition of them being fundamental. Thus, they do not exist out of causal necessity (type 1). They also do not exist inherently (type 3) because they are not tautologies. E.g. Law of Inertia: "An object at rest stays at rest, and an object in motion stays in motion" can be denied without resulting in a self-contradiction. Furthermore, if the fundamental laws of nature existed inherently, this would result in a modal collapse, which is usually frowned upon. Therefore, the existence of the fundamental laws of nature are explained by design (type 2), and for a specific purpose.

The next two groups of things are true only if we presuppose that we have free will. If you don't want to accept that presupposition, then they can be dropped.

2. Man-made things: This presupposes that free will exists, because if not, then we fall back into causal necessity. Whatever is man-made is designed, and designed with a purpose in mind. E.g. a paper-cutter is a man-made device designed to cut paper. A house is a man-made structure designed for humans to reside in. A painting is a man-made thing designed to be appealing to our sense of sight. Etc. There is no man-made thing that is not designed for a purpose.

3. Man itself: A process that is fully determined cannot give rise to a non-determined thing (and quantum should not be used as an objection because even physicists claim they do not fully understand it). Thus, the existence of humans with free will cannot be explained by causal necessity with laws of nature alone (type 1). It is also not explained inherently (type 3) because humans did not always exist. Thus, the existence of humans is designed (type 2). And if so, they have a purpose. Man's purpose is what is usually called the meaning of life.