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The mass-energy content of the Universe is approximately this: 5% normal matter, 20% dark matter, and 75% dark energy. Because we don't know much about dark matter and dark energy, apart from the fact that they should exist in order to explain observations, it is often stated that we only know 5% of our Universe. But isn't this a logical fallacy? This is only the mass-energy content of our universe (as obtained with our current understanding), it is not a degree of how well we understand our universe.

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    Well, your own formulation does not say that we have 5% degree of understanding X, it literally says that we know 5% of X, so 5% modifies exactly what it should. – Conifold Oct 20 '20 at 20:01
  • What is the exact statement that is in dispute here? Is it "We only know 5% of our Universe."? -- a statement that strikes me as barely grammatical. – Dave Oct 20 '20 at 20:14
  • @Dave, "we only know 5% of our Universe" or "we only understand 5% of our Universe". There is this conclusion that I heard many times, that since dark matter and dark energy make up 95% of our Universe, and we don't know much about them, therefore we know/understand mostly nothing about our Universe - we barely understand 5% of our Universe. – AWanderingMind Oct 22 '20 at 8:27
  • To me this argument is not sound. While the mass-energy content is something that can be rigorously and scientifically determined, the degree of knowledge or the level of understanding of our Universe is not something well-defined. And certainly they are not the same thing. – AWanderingMind Oct 22 '20 at 8:30
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Short Answer

This argument hinges upon the notion of what it means 'to know', and it uses know in two different senses; this makes it a fallacy of equivocation.

Long Answer

Merriam-Webster lists a number of definitions of the verb 'to know'. When one begins arguing about knowledge and conflating the different sorts, one equivocates. This is a subtle informal logic fallacy because it's natural to accept both definitions as acceptable and meaningful making it persuasive. In your example:

  1. To know the composition of the universe would appear to be 1a(3) "to recognize the nature of; discern".
  2. To know rules which govern our known universe would appear to be 2b(2,3) "to be acquainted or familiar with", "to have experience of".

So let's simply reformulate your example argument:

P1. We have measured the (physical) stuff of the universe and and according to mathematical models, only 5% is normal matter we can interact with directly with our senses.
P2. We can only understand, be familiar, and have experience of normal matter.
C. Therefore, we can only know 5% of the universe.

There are many problems with this rewritten argument. Does measurement of composition based on mathematical abstraction actually characterize recognizing the nature of something? Must we interact with something to understand it? Perhaps dark matter and energy are a near uniform field and obey deterministic laws yet to be discovered. What does it mean to understand, be familiar, and experience any matter? Even if we can only understand, be familiar, and experience normal matter, the universe is a large place, so how can we claim to have a full knowledge of that 5%? How is using a machine to understand leptons any different than using one to understand dark matter or energy? Do we actually experience leptons?

We could go on, but all of these hinge upon what it means 'to know'. In philosophy, there are two important disciplines of distinctions about knowledge and things called epistemology and ontology, respectively where for thousands of years thinkers have toyed with different ideas of 'to know' and 'to exist'; the argument you have presented is so coarse as to almost be meaningless.

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  • The comments and answer above illustrate the limitation in understanding through direct observation or measurement. Spinoza maintained that only 'scientia intuitiva' a type of knowledge derived through an intuitive grasp of the 'essence' or proximate cause of something beyond direct experience, can result in adequate understanding of, say, the universe. If we define the universe as, 'the encapsulation of the evolution of every possibility in a never ending cycle of recreation'. We can then deduce concrete first principles like, light, mass, gravity and energy to establish a science. See Ethics – user37981 Oct 20 '20 at 18:22
  • @CharlesMSaunders I very much believe in the importance of intuition in understanding, and certainly first principles are a product of them. 'Scientia intuitiva' sounds a lot like what Searle calls the Background. Even measurement and direct observation are themselves normative in some aspects I think you'd agree. – J D Oct 20 '20 at 18:42
  • The incremental accumulation of knowledge which may hopefully lead to some form of 'intuirional' grasp of the 'real', must involve measurement and observation of the sensible world. This certainly points to this world as knowledge-based. If knowledge was only attained in some transcendental plateau without any grounding in experience, then it would be mythical, or mystical and hence unreal. Thanks, J D – user37981 Oct 21 '20 at 2:38
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This seems like a semantic issue to me.

Under an interpretation of "We only understand 5% of our Universe." as "There is only 5% of the universe [the regular matter] of which we have [significant] understanding." seems perfectly reasonable as a qualitative statement about our state of knowledge, and thus not a logical fallacy.

The "5%" is a characterization of the "portion of the universe" of which we have some understanding -- not a description of the amount or quality of the understanding itself.

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