On the supposition that

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1. Definition + Rich Everyday Explanation

On the supposition that (phrase) = assuming that something is true, even if it’s not certain; based on the idea or hypothesis that.

This is a formal way to say “supposing that” or “assuming that” — you’re building an idea, argument, or plan on a particular assumption, treating it as true for the moment.

The core is conditional thinking: you accept a premise temporarily to explore what follows. It’s like saying “let’s pretend this is the case” without committing to it being fact. In real life, people use it to discuss possibilities, make careful predictions, or hedge statements when evidence is incomplete.

“Supposition” itself means an assumption or guess not fully proven. So “on the supposition that” signals caution and logic — you’re reasoning step-by-step, showing you’re aware the base idea might be wrong. It’s common in debates, legal talk, academic writing, or careful planning, where you don’t want to sound overconfident.

In everyday English, it’s quite formal — most natives would say “assuming that” or “if we suppose that” in casual chat, but this full phrase appears in books, articles, or professional discussions to sound precise and thoughtful.

Examples from the street:

• “On the supposition that the train is delayed, we should book a later meeting” → careful planner covering possibilities, shows practicality
• “On the supposition that he’s telling the truth, the story still doesn’t add up” → skeptical friend questioning logic, highlights doubt
• “We’re proceeding on the supposition that funding will be approved” → business talk being optimistic but realistic, manages expectations

2. Most Common Patterns

• on the supposition that + clause → base the idea on this assumption
• proceed/work/act on the supposition that → move forward based on the assumption
• on the supposition + of + noun → slightly less common variant with a noun phrase
• based on the supposition that → similar structure emphasizing foundation
• reject/accept on the supposition that → decide something conditionally

3. Phrasal Verbs

Note: There are no common phrasal verbs directly containing “supposition” — these are related expressions:

• go on → proceed based on an assumption
  Example: “We’re going on the idea that everyone can attend.”
• bank on → rely heavily on something being true
  Example: “Don’t bank on the weather staying clear all day.”
• hinge on → depend entirely on a particular condition
  Example: “The whole plan hinges on getting approval by Friday.”

4. Example Sentences

1. On the supposition that sales increase, we’ll hire more staff→ Assuming revenue grows, the company plans to add new employees.
2. We should plan on the supposition that the event might be cancelled→ Prepare expecting the gathering could possibly not happen.
3. The argument fails on the supposition of perfect information→ The reasoning collapses when assuming everyone has complete knowledge.
4. They’re investing based on the supposition that the market will recover→ Capital is being placed expecting economic improvement soon.
5. I can’t accept the claim on the supposition that evidence is lacking→ The statement remains unconvincing without supporting proof.
6. Let’s proceed on the supposition that everyone agrees→ Move forward assuming full consensus from the group.
7. The budget was approved on the supposition that costs stay stable→ Funds were allocated expecting expenses to remain consistent.
8. Critics attacked the theory on the supposition of flawed data→ Opponents challenged the idea assuming inaccurate information.
9. We acted on the supposition that delivery would be on time→ Decisions were made expecting timely arrival of goods.
10. The decision rests on the supposition that risks are minimal→ The choice depends on believing dangers are very low.

5. Personal Examples

1. Teachers often plan lessons on the supposition that all students have completed homework→ Instructors prepare classes assuming every learner finished assigned tasks.
2. When improving speaking skills, progress fastest on the supposition that mistakes are normal and helpful→ Language fluency advances quickly when accepting errors as natural learning steps.

6. Register: Formal

✔ Native usage tips

• Natives use this phrase in writing or careful speech to sound logical and precise — like in emails: “On the supposition that the deadline holds…” — shows thoughtful planning.
• In casual talk, people shorten to “assuming that” or “supposing” — the full phrase feels academic or professional.
• Common in debates or analysis: “On the supposition that X is true, then Y follows” — classic reasoning structure.
• It softens statements — using it shows you’re not claiming absolute truth, just exploring possibilities.

✔ Similar expressions / words

• Assuming that → more common and neutral; everyday alternative
• Supposing that → slightly more hypothetical; often introduces questions
• On the assumption that → very close, perhaps even more common in business