It may seem reasonable to trust in only those things detectable by the senses. However, upon serious reflection, this position lacks the explanatory power to account for even the most fundamental realities of life.
The viewpoint that “seeing is believing” assumes that all reality is comprised of physical matter. This idea represents a crass form of materialism. In other words, for something to exist, it must be visible. And if it’s visible to the eye, then it must be physical in nature.
Yet many things that actually exist cannot be observed. Some very real scientific entities, such as magnetism, gravity, and electricity cannot be seen. Critical abstracts, such as numbers, sets, propositions, and properties, cannot be seen either. Yet they exist. Justice, goodness, and other morals are real, but they are also undetectable by the senses. The concepts of truth and love are invisible realities. Feelings and emotions surely exist although they cannot be observed in a direct manner.
A central weakness of all forms of materialism is this inability to account for the transcendent components of life. This defect is uniquely true in relationship to cognitive or conceptual elements. We cannot see our thoughts, and although the brain is observable, the mind is not. And yet we don’t doubt the reality of either. For a person to deny the existence of the mind and thoughts because they are unobservable is considered irrational. Are the formal laws of logic physical, material, or visible? No. Yet they are realities—the very anchors that make human rationality and the world’s intelligibility possible.
On the other hand, belief in the invisible biblical God is justified. His essence can’t actually be observed. However, His existence can be inferred as a reasonable explanation for the necessary realities of life, including the universe, abstract entities, ethical principles, human beings, and religious phenomena.
This belief is not a naively assumed “god-of-the-gaps” position but, unlike materialism, a genuine and valid explanatory perspective on the nature of reality.
Resources: See Kenneth’s book Without a Doubt. For a summary of the book’s contents, listen to the Without a Doubt podcast, available for free via iTunes.