Defenses

Obviousness under § 103

Most patented inventions are combinations of existing ideas. The modern KSR standard makes those combinations easier to attack than they used to be.

5 min read · Updated Apr 29, 2026

35 U.S.C. § 103 is the most-used invalidity defense in patent litigation. It's also the most-misunderstood. § 102 anticipation requires a single reference disclosing every limitation. § 103 obviousness lets you combine multiple references. That sounds easier — and it is — but the rules are subtle.

The modern framework comes from KSR International v. Teleflex (Supreme Court, 2007). It replaced an older, more rigid test (the "TSM test" — teaching, suggestion, motivation) with a flexible analysis that has since shaped every § 103 case.

What "obvious" means legally

A claim is obvious under § 103 if "the differences between the claimed invention and the prior art are such that the claimed invention as a whole would have been obvious before the effective filing date to a person having ordinary skill in the art (PHOSITA)."

Key concepts:

  • PHOSITA. A hypothetical person with ordinary skill in the relevant field. Not a genius, not a layperson. Someone who knows the prior art and the standard tools but doesn't have insight beyond that.
  • As a whole. The claim must be obvious as a whole, not just element-by-element.
  • At the time. Judged with the knowledge available before the filing date. No hindsight allowed (in theory).

The KSR framework

KSR gave courts and the PTAB a flexible test:

  1. Identify the scope and content of the prior art.
  2. Identify the differences between the claim and the prior art.
  3. Determine the level of ordinary skill in the field.
  4. Resolve secondary considerations — commercial success, long-felt need, failure of others, copying, unexpected results.
  5. Combine, with motivation. A claim that combines elements from two prior-art references is obvious if a PHOSITA would have been motivated to make the combination — and would have had a reasonable expectation of success.

The motivation to combine is where most § 103 cases are won and lost.

Motivation to combine

Pre-KSR, courts required an explicit teaching, suggestion, or motivation in the prior art. Post-KSR, motivation can come from:

  • Common sense. A PHOSITA would obviously try this combination.
  • Predictable variations. Applying a known technique to a known problem to yield predictable results.
  • Design needs and market pressures. Incentives to improve a known product.
  • The nature of the problem. An obvious fix for an obvious problem.
  • Explicit teaching in the prior art (the old TSM test, still valid as one path).

The flexible standard makes obviousness a powerful defense. Most claimed inventions are combinations of existing techniques; the question is whether a PHOSITA would have made the combination.

What's NOT enough

Common patentee responses that often fail:

  • "Hindsight reasoning." The patentee argues the defendant only sees the combination as obvious because the patent points it out. Courts reject this when there's an independent rationale — the prior art itself suggests the combination.
  • "Teaching away." A reference that affirmatively discourages the combination weighs against obviousness — but only if it really teaches away, not just describes a different option.
  • "Unexpected results." A patentee argues the combination produced surprising benefits. Powerful when supported by data; usually not.

Secondary considerations (objective indicia of non-obviousness)

The Graham v. John Deere (1966) factors, still alive post-KSR:

  • Commercial success. Does the product practicing the patent sell well?
  • Long-felt but unsolved need. Was there a known problem that nobody else had solved?
  • Failure of others. Did competitors try and fail at the same thing?
  • Copying. Did the accused infringer copy the invention specifically?
  • Praise by industry. Did experts call it innovative?
  • Skepticism. Did the field doubt the approach?

Patentees rely on these heavily. They're real but rarely outcome-determinative — a strong prior-art combination usually beats commercial success.

Building an obviousness case

Practical workflow:

  1. Identify the closest single reference. This is the foundation.
  2. Identify the missing limitations — what does the closest reference lack?
  3. Find a second reference that supplies the missing limitations.
  4. Articulate motivation to combine. Why would a PHOSITA, in the relevant field, have combined these references? Be specific.
  5. Articulate reasonable expectation of success. A PHOSITA wouldn't combine references that wouldn't predictably work together.
  6. Anticipate the patentee's response. Identify the strongest secondary considerations and rebut them.

A good § 103 case usually combines two references — sometimes three. Beyond three, courts and the PTAB get suspicious of "selecting and combining" hindsight.

KSR in practice

Examples of § 103 obviousness arguments that courts and the PTAB find persuasive:

  • Applying a well-known technique from one field (e.g., signal processing) to a closely-related field (e.g., audio) — usually obvious.
  • Combining two references that solve the same problem in similar ways — usually obvious.
  • Substituting one known element for another (a Bluetooth radio for an infrared one) — almost always obvious.
  • Adding a generic computer to a known process to "automate" it — very obvious post-Alice; also vulnerable under § 101.

Less likely to succeed:

  • Combining references from very different fields with no reason for a PHOSITA to have looked across them.
  • A combination that requires the PHOSITA to reverse-engineer the claimed invention to work.
  • A combination that the prior art expressly discourages.

In litigation vs. IPR

District court requires "clear and convincing" evidence — same as § 102.

IPR uses the lower "preponderance" standard. § 103 is a frequent IPR ground; combinations of patents and printed publications are exactly what IPR is designed for.

Most IPR petitions assert both § 102 (for the simplest claims) and § 103 (for everything else). When the defendant has good prior art, both grounds usually live or die together.

Bottom line

§ 103 is the workhorse. Most patented inventions are combinations of existing ideas, and the modern KSR standard makes those combinations easier to attack than they used to be. The key is finding two solid references and articulating a non-hindsight reason a PHOSITA would have made the combination. Patent defense is, in large part, the craft of building § 103 arguments. (See a worked § 103 analysis on the Lumen View patent.)

This article is for general education and is not legal advice.