Here's a super-useful conversation I had with Chat-GPT about the one area where these two gods of marketing disagree:

AI: So on your Rolls-Royce/SKoda example, Sharp would say: of course they're not interchangeable - they're effectively in different categories because of price. His challenge is directed at marketers who then conclude that, within the mainstream car market, every brand needs a unique psychographic niche. The evidence from the Ehrenberg-Bass Institute suggests that buyers of competing brands within the same competitive set are much more alike than different, and long-term growth usually comes from attracting more of all category buyers rather than appealing ever more strongly to a narrowly defined segment.

Me: So it all depends on how you define your market? Isn't this a circular argument... whenever brands do not share potential buyers Sharp will say 'ah yes, because they are in different markets'?

AI: I think you've identified what is probably the strongest philosophical criticism of Sharp's work. Marketing academics have made exactly this point.
The answer is yes, to an extent. Everything depends on defining the competitive market correctly. The interesting question is whether Sharp's definition is independently observable or whether it's defined after the fact.

Sharp's defence
Sharp would reject the accusation of circularity because he doesn't define markets by marketing strategy or brand positioning. He defines them empirically by asking questions like:
Which brands are actually substituted for each other?
- Which brands share buyers?
- Which brands compete for the same purchase occasions?
- Which brands exhibit the same buying patterns (e.g. the law of duplication of purchase)?
 -If two brands consistently share buyers, they're in the same competitive market. If they don't, they're probably not.
That's a testable proposition.

But critics say...
The criticism is that this approach can become tautological.
Suppose you observe that Rolls-Royce buyers rarely buy Skoda.
Therefore they aren't in the same market.

Someone might reply:
If you define the market by observed substitution, then many strategic questions can never be answered.
For example:
- Could Skoda create a premium sub-brand?
- Could a luxury EV manufacturer disrupt traditional luxury?

Sharp's framework is much stronger at explaining how mature markets behave than predicting when market boundaries might shift.

Ritson's view
This is where Ritson thinks STP (Segmentation, Targeting, Positioning) still earns its keep. He'd argue that firms make strategic choices about:
- which competitors they want to face,
- which price points to occupy,
- which occasions to emphasise,
- which consumers to appeal to.
Those choices help define the market. So rather than saying: "Observe the market." Ritson says: "Part of strategy is deciding which market you're going to compete in and how you are going to shape the market".

A good example: Tesla
Consider Tesla in 2012.
If you'd applied Sharp's framework then, you might have concluded:
- luxury EVs aren't really a market;
- Tesla has very few shared buyers;
so, the category boundaries are unclear.

But strategy created a new competitive space. Today Tesla competes with:
- BMW
- Mercedes-Benz
- Audi
The market itself evolved.

Sharp would probably say:
Fine, that's exactly what happened. The empirical market changed.

Critics respond:
Yes - but strategy caused the change, so simply observing today's buying patterns wouldn't have predicted it.

Where I think Sharp is strongest
Sharp is extremely persuasive in saying:
Within an existing competitive market, marketers exaggerate differentiation.
For example, if you're marketing lager in the UK, it's difficult to argue that each brand serves a unique psychographic tribe. Buyer overlap is substantial, and the empirical laws he describes are robust.
Where he's weaker
He's less convincing on questions like:
- Could/when should a brand move upmarket?
- Could/when should it create a new category?
- How do categories themselves evolve?
- How does disruptive innovation redefine substitution?
These are questions about competition and strategy, not just buying behaviour.

My preferred synthesis
I think the two perspectives fit together rather well:
- Sharp explains market physics. Given a market as it exists today, his laws tell you how brands typically grow: by increasing mental and physical availability, reaching more category buyers, and building distinctive memory structures.
- Strategy explains market geometry. It asks whether you should redefine the market, move into a different price tier, create a new category, or change the basis of competition.

An analogy might help. Sharp is like a physicist explaining the laws of aerodynamics once an aircraft is in flight. Ritson (or more broadly, strategic marketing thinkers) is more concerned with deciding what kind of aircraft to build and where to fly it.

So your concern about circularity is valid if Sharp's framework is used to answer all strategic questions. It's much less problematic if you see it as describing how brands compete within a defined competitive arena, while recognising that defining - and sometimes reshaping - that arena is itself a strategic act.