diff --git a/packages/safe-ds-lang/src/language/builtins/packageNames.ts b/packages/safe-ds-lang/src/language/builtins/packageNames.ts
index b6abe37d9..4e225348b 100644
--- a/packages/safe-ds-lang/src/language/builtins/packageNames.ts
+++ b/packages/safe-ds-lang/src/language/builtins/packageNames.ts
@@ -1 +1,2 @@
 export const BUILTINS_ROOT_PACKAGE = 'safeds';
+export const BUILTINS_LANG_PACKAGE = `${BUILTINS_ROOT_PACKAGE}.lang`;
diff --git a/packages/safe-ds-lang/src/language/validation/inheritance.ts b/packages/safe-ds-lang/src/language/validation/inheritance.ts
index 3e89a4c0e..a5e382df8 100644
--- a/packages/safe-ds-lang/src/language/validation/inheritance.ts
+++ b/packages/safe-ds-lang/src/language/validation/inheritance.ts
@@ -41,7 +41,7 @@ export const classMemberMustMatchOverriddenMemberAndShouldBeNeeded = (services:
             );
         } else if (typeChecker.isAssignableTo(overriddenMemberType, ownMemberType)) {
             // Prevents the info from showing when editing the builtin files
-            if (isInSafedsLangAnyClass(node)) {
+            if (isInSafedsLangAnyClass(services, node)) {
                 return;
             }
 
@@ -54,9 +54,12 @@ export const classMemberMustMatchOverriddenMemberAndShouldBeNeeded = (services:
     };
 };
 
-const isInSafedsLangAnyClass = (node: SdsClassMember): boolean => {
+const isInSafedsLangAnyClass = (services: SafeDsServices, node: SdsClassMember): boolean => {
     const containingClass = getContainerOfType(node, isSdsClass);
-    return isSdsClass(containingClass) && getQualifiedName(containingClass) === 'safeds.lang.Any';
+    return (
+        isSdsClass(containingClass) &&
+        getQualifiedName(containingClass) === getQualifiedName(services.builtins.Classes.Any)
+    );
 };
 
 export const classMustOnlyInheritASingleClass = (services: SafeDsServices) => {
diff --git a/packages/safe-ds-lang/src/language/validation/names.ts b/packages/safe-ds-lang/src/language/validation/names.ts
index 5a9d43e6a..c529efcb6 100644
--- a/packages/safe-ds-lang/src/language/validation/names.ts
+++ b/packages/safe-ds-lang/src/language/validation/names.ts
@@ -1,7 +1,7 @@
 import { AstNodeDescription, getDocument, ValidationAcceptor } from 'langium';
 import { duplicatesBy } from '../../helpers/collectionUtils.js';
 import { listBuiltinFiles } from '../builtins/fileFinder.js';
-import { BUILTINS_ROOT_PACKAGE } from '../builtins/packageNames.js';
+import { BUILTINS_LANG_PACKAGE, BUILTINS_ROOT_PACKAGE } from '../builtins/packageNames.js';
 import {
     isSdsQualifiedImport,
     SdsAnnotation,
@@ -46,6 +46,7 @@ import { SafeDsServices } from '../safe-ds-module.js';
 import { declarationIsAllowedInPipelineFile, declarationIsAllowedInStubFile } from './other/modules.js';
 
 export const CODE_NAME_CODEGEN_PREFIX = 'name/codegen-prefix';
+export const CODE_NAME_CORE_DECLARATION = 'name/core-declaration';
 export const CODE_NAME_CASING = 'name/casing';
 export const CODE_NAME_DUPLICATE = 'name/duplicate';
 
@@ -68,6 +69,36 @@ export const nameMustNotStartWithCodegenPrefix = (node: SdsDeclaration, accept:
     }
 };
 
+// -----------------------------------------------------------------------------
+// Core declaration
+// -----------------------------------------------------------------------------
+
+export const nameMustNotOccurOnCoreDeclaration = (services: SafeDsServices) => {
+    const packageManager = services.workspace.PackageManager;
+
+    return (node: SdsDeclaration, accept: ValidationAcceptor) => {
+        if (!node.name) {
+            /* c8 ignore next 2 */
+            return;
+        }
+
+        // Prevents the error from showing when editing the builtin files
+        const packageName = getPackageName(node);
+        if (packageName === BUILTINS_LANG_PACKAGE) {
+            return;
+        }
+
+        const coreDeclarations = packageManager.getDeclarationsInPackage(BUILTINS_LANG_PACKAGE);
+        if (coreDeclarations.some((it) => it.name === node.name)) {
+            accept('error', 'Names of core declarations must not be used for own declarations.', {
+                node,
+                property: 'name',
+                code: CODE_NAME_CORE_DECLARATION,
+            });
+        }
+    };
+};
+
 // -----------------------------------------------------------------------------
 // Casing
 // -----------------------------------------------------------------------------
diff --git a/packages/safe-ds-lang/src/language/validation/safe-ds-validator.ts b/packages/safe-ds-lang/src/language/validation/safe-ds-validator.ts
index a7be8a0b5..26b405ed2 100644
--- a/packages/safe-ds-lang/src/language/validation/safe-ds-validator.ts
+++ b/packages/safe-ds-lang/src/language/validation/safe-ds-validator.ts
@@ -51,6 +51,7 @@ import {
     functionMustContainUniqueNames,
     moduleMemberMustHaveNameThatIsUniqueInPackage,
     moduleMustContainUniqueNames,
+    nameMustNotOccurOnCoreDeclaration,
     nameMustNotStartWithCodegenPrefix,
     nameShouldHaveCorrectCasing,
     pipelineMustContainUniqueNames,
@@ -231,6 +232,7 @@ export const registerValidationChecks = function (services: SafeDsServices) {
         SdsClassMember: [classMemberMustMatchOverriddenMemberAndShouldBeNeeded(services)],
         SdsConstraintList: [constraintListsShouldBeUsedWithCaution, constraintListShouldNotBeEmpty],
         SdsDeclaration: [
+            nameMustNotOccurOnCoreDeclaration(services),
             nameMustNotStartWithCodegenPrefix,
             nameShouldHaveCorrectCasing,
             pythonNameShouldDifferFromSafeDsName(services),
diff --git a/packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name with block lambda prefix.sdstest b/packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name with codegen prefix.sdstest
similarity index 100%
rename from packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name with block lambda prefix.sdstest
rename to packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name with codegen prefix.sdstest
diff --git a/packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name without block lambda prefix.sdstest b/packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name without codegen prefix.sdstest
similarity index 100%
rename from packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name without block lambda prefix.sdstest
rename to packages/safe-ds-lang/tests/resources/validation/names/codegen prefix/package name without codegen prefix.sdstest
diff --git a/packages/safe-ds-lang/tests/resources/validation/names/core names/in safeds lang.sdstest b/packages/safe-ds-lang/tests/resources/validation/names/core names/in safeds lang.sdstest
new file mode 100644
index 000000000..e3a02f0a0
--- /dev/null
+++ b/packages/safe-ds-lang/tests/resources/validation/names/core names/in safeds lang.sdstest	
@@ -0,0 +1,4 @@
+package safeds.lang
+
+// $TEST$ no error "Names of core declarations must not be used for own declarations."
+annotation »Number«
diff --git a/packages/safe-ds-lang/tests/resources/validation/names/core names/main.sdstest b/packages/safe-ds-lang/tests/resources/validation/names/core names/main.sdstest
new file mode 100644
index 000000000..78ccb7d08
--- /dev/null
+++ b/packages/safe-ds-lang/tests/resources/validation/names/core names/main.sdstest	
@@ -0,0 +1,7 @@
+package tests.validation.names.coreNames
+
+// $TEST$ error "Names of core declarations must not be used for own declarations."
+annotation »Any«
+
+// $TEST$ no error "Names of core declarations must not be used for own declarations."
+annotation »Any1«